First of all, this is really long so I hope it's okay. Second of all, the first step in this strategy all depends on how much $$$ you start out with. If you start out with 10,000 or less, then do not follow the first step. 30,000 or 50,000 starting cash is okay.
First step: First, select your dozer or worker that you started out with. Tell him to build a power plant if US or China, or a barracks if GLA. Then, select your Command Center. Build 9 construction units, whatever they are depending on which army you are. As soon as each one is produced, send them to a supply source, but alternate which supply source with each unit.
Step for those who started out with 10 K or less: Only build 2 construction units. Send each one to a supply source. Then follow the rest of the first step.
Second step: When your construction units reach the supply sources, have them build defenses. If you are the US, you should start by building 3 Patriot missiles and 2 fire bases at each extra Supply source, but keep the fire bases un-garrisoned until you have enough generals points to have Pathfinders. Then fill 'em up with pathfinders. If you are China, 3 gattling cannons and 2 bunkers full of tank hunters should suffice.
If you are GLA, 5 stinger sites, 2 tunnel networks. Make sure all have camo netting as soon as possible.
Third Step: Now, fortify your main base a lot with defenses--twice the amount that are at your supply sources. Climb the tech tree as soon as possible and obtain a superweapon or powerful generals ability as soon as possible. Your opponent, if he has half a brain, should be trying to throw everything he's got at your supply bases. Your defenses should be chewing him up real good. If he starts using artillery, respond with your own artillery to take out his. (Tommahawk Cruise Missile Launcher, Nuke Cannon, Scud Launcher, depending on which side you are.) If he still manages to destroy your defenses (unlikely) then sell all your buildings at your bases and retreat to your main base.
Fourth Step: If you still have your supply bases, begin building production centers at them. (barracks, war factories/arms dealers.) Use these and the no-doubt $$$ you've gained from your supply bases to build a HUGE army. If you haven't amassed that much, then build secondary supply buildings at your main base (USA secondary supply source, for example, is supply drop zones) It doesn't matter what units you build, just as long as you build a whole lot. If you are one of the generals, build whatever special units that general has. Send this army to his base--try to flank him on a spot where he doesn't have so many defenses, if such a spot exists. If he is totally fortified to the teeth, use a superweapon to take out his defenses. Your army should be more than enough to take him out, if it is large enough. There is one golden rule in this great game--no matter how big your opponent's army is, if yours is twice as big, you'll win. This strategy takes really long and requires a lot of patience. It will always work against a hard computer opponent or less, and it will almost always work against a human opponent unless he is really good. Thank you for reading this and I hope this strategy works. (I've won 10 times straight using it.) This of course, will only reach you if this site decides to post this really long strategy.