You must move forward, establish defenses as far forward from your base as you can, at natural choke points in the map so that you can concentrate the enemy into a small space and hammer him with your defenses. The thing to remember, if you are only concentrating on defending your base (waiting for research before building units, etc), the next thing you will know is that the enemy is at your doorstep and it's all over bar the singing. From your first post I can see you know the feeling of helplessness when the enemy is outside your base, surrounding you and you have nowhere to go...
In light of this, it is very rare that I will fortify my actual base very much - maybe at the start of the game, just enough to withstand a rush and mg towers will normally stop this. What I do is after first getting my basic base up and running is; I move some trucks as far forward as I can, trying to make the center point between myself/the enemy and then start building my defenses there while moving my combat units up as they are produced (you will see later why you should protect these trucks with the first combat units you are producing).
With these combat units I will start sending out feelers to gauge what the enemy is doing - you catch his trucks, you kill them straight away as this will impede his defenses as he is probably trying to do what you are (it is worth noting that killing trucks is of value enough to warrant sacrificing units, the more trucks you can kill the better because it disrupts the enemies plans in a big way). Keep moving forward until you encounter significant resistance - if you don't encounter anything just keep moving forward, you may have been lucky and caught him with his pants down and you can move straight to his base. Also take into account that an encounter with an enemy can prompt him to all out attack - this can be an advantage or a disadvantage depending on what defenses you have up and the size of your army - if you can gode him into an attack before he is prepared and draw him into your defenses all the better... if however you are not prepared, well this might not be so good...
When you do encounter significant resistance you may need to back off - if you have more units than the enemy you may be able to beat him, if not just back off to your forward defenses. In regards to tactics and engaging the enemy, I like to use a two step attack - first off I send cyborg flamers in to block the enemy and munch on his cyborgs and tanks, followed closely behind by my main tank group (close means like probably 5-10 tank lengths - you need to be able to escape his flamers if required, yet close enough to be able to fire at his units engaging your borgs (always set your firing range to long range). If the enemy has his borgs and tanks all together they will all be either destroyed or significantly weakened by your borg flamer attack, then you can mop them up with your tanks - your greatest danger at the first TI stages of the game is flamers / cyborgs - you need your cyborg flamer screen to counter this. Always retreat your tanks from flamers early in the game, they have a short range so if you have like machine guns they will kill the flamers while they are chasing you - the thing is that even a small amount of flamers can damage your tank group badly, remember they are not just attacking one tank, the flames are burning everything in their line of fire!
Side Note: if the game is at later stages, your screen will become heavy units, like cannon protecting a fast attack force of lancer hovers or other such strong attack/weak defense units. The heavy units block the enemy from attacking your weak lancers while your lancer fire thru your heavy units and devastate the enemy. You can also put in some mortor or artillery in this fast attack force to work on enemy defenses - all protected by your heavy units! Be careful though with mortar and artillary - they need to stop to fire and they also have area attacks so they can hurt your heavy screening units if the enemy is close to them - probably best to group these together and keep them out until you need to work on enemy defenses - cb and senor turrets can come into their own here. Also learn exactly how to research ripple rocket battery http://guide.wz2100.net/r/ripplerocketbattery, and concentrate on getting these as fast as possible, they are a game winner for later stages - you should mass build ripple rocket emplacements when they come available then you can use sensor turrets to creep around and pound at your enemy - ripples have a VERY long range and are the earliest long range weapon that you can get - the only other batteries that compare (in regards to range) are the ground shaker and arch angel missiles which take a LOT more time to research!
Ok back to what I was talking about before... If the enemy is doing the same thing and you have to back off, try and draw him into the defenses you have made, at this stage it is not about blocking him, it's about weakening his army without losing your army - ie you don't protect your defenses with your units, you protect your units with your defenses. In light of this I like to build defensive patterns that draw the enemy into channels so he keeps moving forward and getting hit - I usually use parallel lines to achieve this (can be mg bunkers, hardpoints, whatever):
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
0 0 0 0 0
as opposed to one line blocking the chokepoint or whatever
see with the second type of wall type defense, the enemy breaks thru and he is thru without further resistance, with the first channel type setup he will just keep moving forward and if he tries to rush thru his units are gonna suffer - and hopefully you have your fresh army waiting at the end for him
. Another thing with parallel channels, it does not impede your own army, you can move them thru the channels easily (or retreat thru the channels as well if you need). Remember that you defenses can impede you just as much as the enemy if not set up right (more so because you cannot blow them up like he can
- never completely block off an area with defenses unless you are absolutely sure you won't need to move units thru them. At times you can have 100+ units so if you have made your gate or area too narrow you won't be able to get thru fast.
Remember if he is rushing your defenses he is probably using the bulk of his army to do so, if you destroy his army, don't sit back, move forward, capitalize and if you can go right to his base (but don't get caught the same way he did
- but then again if you have killed most of his army, he won't have an army waiting for you if you break his line - so in that case go and win your game
If his defenses are a bit strong yet, mass your army just outside their range and bring your trucks up and create a new defensive line (always moving forward remember!) - you should have mortar pits by now, whatever the case keep the ground you have gained (always try to retain the ground you have gained) and know when to stop pushing forward, you don't want to lose your army!
Remember first concentrate on getting your base built up to what you want so you can research and get power / factories etc - you don't want to be distracted by your base while working in your forward area, so try to get this set up as much as you can quickly, use cuing, etc.
Also that normally the biggest army will win, so mass your units - the best starting units I find is twin machine gun viper half tracks and cyborg flamers you send a massed army of these at the enemy early and he could be in deep trouble - this is why it is important not to wait too long before producing units, a hundred viper halftrack twin mg is gonna easily beat a handful of heavy body units, and that is all you will have if you have waited for the research to build these units.. neglecting to build any early units. Even waiting for twin mg and half tracks is a small gamble - an opponent working a rush so much that he spends all his early money on creating basic mg viper wheels could have a substantial amount of units coming at you by the time you get to twin mg - this does occasionally happen and if you are not prepared you are a goner - if you manage to repulse the attack you are in very good position to counter attack as he has neglected building up his base and researching to do the rush so you will be at an advantage.
Also if you encounter light defenses and you have a huge army, don't be afraid of them and back off, you can lose an important gain like this - just go thru the defenses and straight for his base - remember not to get caught in parallel channels like mentioned above, if he just has one line with some machine gun bunkers, etc just mass your units in one point and make a hole then go straight thu.
This advice is probably more for high oil mp maps (which I play the most), but the principles can work in low oil - remember in low oil it's about getting to the derricks early and protecting them (a little) but still massing your army and forward defensive lines.
oh and always evolve your tactics, if the enemy does something that really messes with you, integrate it to your tactic, use this knowledge in your next game, learn from your mistakes capitalize on your victories. Unlike real wars we are lucky enough to get another go at it
Disclaimer - I am not a pro, just sharing some things that have worked for me and stopped me getting spanked like a noob (well some of the time hahahaha).
Well I hope this is useful - ended up becoming more involved that I originally intended....