At last you can do some real fighting. You start with two SCVs. Order them to harvest minerals and begin training five more at your command center. Take Raynor and the marines to the northeast of your base where the only entrance is to protect it from zerg while you wait on your military to begin production. The first four SCVs you produced should harvest minerals. The fifth will start a refinery on the vespene geyser to the north. When you're able, build two more SCVs to harvest gas and finally build one more to build some additional buildings at your base. Build an engineering bay first. You'll be able to upgrade your units' armor and weapons. Let that SCV then harvest gas. Train enough marines until you have six of them total and Raynor. Then build a few more to guard your base while Raynor and the marines go on an expedition to kill some zerg.

Begin your attack to the northeast of your base where there is a small group of zerg. Kill them then backtrack and take the southeastern ramp to go up to a higher elevation. You'll encounter several zerg here and as you explore will find there is only so far you can go. At the end you'll encounter your first two hydralisks who are more threatening than the zerglings. Kill them off and explore as much as you can then go back off the ramp and take the northwestern one this time. You'll see a larger group of zerglings and your first creep colony. Zerg can only build on the creep so destroying this will ensure they can't build anything else there. Northeast of there is a barracks out of which two SCVs will emerge. Set them to gathering minerals back at your base. A little north and you gain control of an entire Terran base. You also get an academy and can train firebats now. They're good for close range fighting and do some high damage. Add a few of them to your group and begin producing firebats and marines from the two newly acquired barracks. Fill up the bunkers to the east and they'll protect your base. Rsearch the U-238 shells upgrade to increase the range of your marines' guns. You can also research stimpacks now but I recommend not using that as it damages your units and is best used when you have access to medics to heal them afterward. Also you can build comsat stations at your command centers to scan unknown territory. Reconnaisance is always useful in real time strategy games. Now continue eastward with your assault force.

You'll find an even larger group of zerglings and another creep colony here. They shouldn't pose a threat to your troops but if they kill off too many of your men you can always replace them. Destroy the creep colony then head southeast down the ramp. You'll find two more hydralisks and a zergling here. Kill them and head south. Take out the small groups of zerglings you encounter and follow the path around to the west. When all the resistance around this area is eliminated go back to the north and up the northeastern ramp. You'll find the lightly defended zerg base here. Destroy all the hydralisks and zerglings first then focus your fire on the creep colonies and, finally, the infested command center.