While most people argue against using walls in MP, I definitely suggest using them against the computer, especially if you're playing against 7 hard opponents. The computer's notorious for dashing itself against walls and getting slaughtered by the turrets. Even the computer's airforce will concentrate on walls to let in the ground forces instead of attacking buildings and units (unit they're right next to the wall).
Usually with the computer I'll play heavy defensive at first. The computer usually won't attack you in Tech I so focus most of your time on resource gathering. It doesn't hurt to build a few troopers to kill off scouts, but don't look to build an invasion force, unless you're playing on a smaller map and there's someone practically on top of you. Even if this is the case you might want to avoid them if you're playing against a lot of opponents by yourself. Otherwise you might get swamped by the other computer players.
Similarly, most people will tell you to get at least 25-30 workers before advancing to Tech II. If you can do this, great. If the computer starts jumping the gun and hitting Tech II when you only have 15 workers, you might want to consider making the jump early as well. Otherwise you're liable to find yourself flooded with troopers and trying to hold them off with recruits.
At Tech II, go ahead and build walls around your area, focusing primarily on the choke points so you can keep your ore costs (and time costs) to a minimum. If you're lucky, you will have found 1-3 holocrons by now and will probably want to build a padawan or two to capture them. Padawans are decent fighters so it's handy to have them around in case you get push hard anyway. Go ahead and fill your turrets with troops, so you'll have them in case someone does manage to break through. I usually don't advance much in the way of upgrades in Tech II, since I tend to attack at Tech III. What I usually do advance is my troopers, carbon mining, food (farms and/or animal shelter, depend on how I'm primarily getting my food), and the range boost for turrets/ships/CCs/Fortresses. I may advance others, but that depends on how heavy I'm being pressed. Also, if you're playing on a water heavy map; Tech II is a good time to build and send out light frigates. They're handy for taking out any food gatherers in the water, and for slaughtering troops, especially if you have the distance upgrade. If you have early control of the water, it makes using cruisers more viable later as well.
In Tech III, I'll usually start developing my air fast. 3-4 fighters and 2 bombers works quite well to level a nearby enemy base since the computer can often be weak in AA during the early air rush. I focus on powercores and animal shelters first to cut military output and food intake (that and I'd rather have the animals for my own). Sending in some strike mechs is also helpful, since they can take troops and workers down very quickly. While I'm striking my opponent, I'll usually have workers on the way so I can take over the base of my opponent and give myself more room for collecting resources. This can be especially useful since if you can keep most of the computer's walls, they work quite effectively against opponents (who can't shoot them). Simply eliminate the gate and replace it with your own. Of course, any turrets/Fortresses you build will slowly destroy the walls, but you can simply replace them as needed.
Most of my early upgrades in Tech III have to do with either increasing my resource intake or building my defenses (especially anything that upgrades my turrents/Fortresses). From there it's a matter of what the opposition is like when I decided whether to continue researching of jump to Tech IV .