There are fortunately only two entrances to your base. You'll want to get a total of eight SCVs working on minerals and three on vespene gas. I recommend building additional bunkers at the two entrances to the base. Don't neglect firebats as they can shoot outside the gunnery holes in the bunkers and damage zerglings that attack. So I guess I recommend one firebat and three marines in each bunker. Upgrade your unit firepower and the marines' attack range to help them pop the hydralisks that will inevitably attack. Also build decent coverage of missile turrets so you can shoot down the mutalisks (flying creatures) the zerg now posess. You don't need to go overboard as most of the enemies are going to come from the land but keep a nice area of cover to the northeast as that is where these creatures will come from. Also, you can finally build factories to produce vultures.

The frequency and strength of the zerg attacks will increase as time passes. Keep your defensive zones reinforced and your bunkers repaired. You could have an SCV on each side of your base to repair damage to your bunkers after each wave. Research the vultures' spider mines ability and place some inside your base behind the entrances should the zerg break through with ground forces. I also kept a group of marines inside my base should the zerg manage to get past the spider mines. I had them hotkeyed and ready to use stimpacks to respond quickly and put down the attackers in a sort of kamikaze attack. And they were somewhat useful. At around two minutes remaining the zerg will launch a massive assault, likely breaking through your lines. Try to do as much damage to them as possible so it will take them a while to destroy your base. You just have to get to 0:00 and you'll be victorious.