this usually works against humans, but won't do much good against comp.

first build a couple command centers,as this strategy can be costly. get at least ten scv's mining minerals, and four or five collecting gas.
get at least six bunkers around the front of your base and load them with firebats, to guard against the rush attacks that happen so much when playing on battle.net. you'll need to get them quickly, so that your base isn't left unguarded. then build an academy and a couple engineering bays, a factory or two, and two starports with com towers. use the starports to get about 24 wraiths, and make sure they have their reactor and the cloaking ability. with the engineering bays, keep upgrading weapons first. as for the academy, research long range shells first. make about twelve seige tanks with the factories, so build a machine shop and research seige mode

then, keep building bunkers a distance from the last, but not too far apart. then load the front bunkers with two marines(or ghosts) and two firebats, so they can stand close range attacks better. keep advancing the units from the last line of bunkers to the next, and keep loading the second to the first bunkers so it looks like every bunker is full. try to keep some seige tanks in seige mode between the bunker lines, so they can use their shock cannons. after this strategy starts to get less effective, send those wraiths in and start making the seige tanks take out the enemy missile turrets (if any) if there arent any, send them against the enemy's barracks gateways, etc. (this last part works especially well against zerg, sice everything comes from the hatchery) soon the battle should be won and you victorious!

CreepColony's Opinion: It might be hard to do this so late in the game, and I would suggest putting all marines in your bunker, unless they go mass zerglings or zealots.