Unload all your SCVs from the command centers and start mining. Land one as close as you can, and hover the other two over the mineral patches you are mining. Now lift them all off and move them to a mining zone. Since you do not have Medivacs yet, an easy way to conduct mining operations quickly in enriched zones is to build three more Command Centers (two if you find the missing miners, or one if you are willing to risk your starting Command Center) and load five SCVs into each, filling them. Therefore at least two Bunkers, one at every entrance, with the support of the units you produce, are recommended. Zerg attacks come from different directions. Still, you should not neglect defending your main base. Retrieving these resources is very beneficial, as they amount to several thousand of resources and taking down the bases is very manageable, even on Brutal. You will also find a lot of mineral and gas containers which also appear when destroying certain Zerg buildings. This will not only stop the Zerg attacks, but you'll find numerous Reaper groups along the way.

The game instructs you to just defend your mining operations, but you can and probably should build up a sizable force of Marine/Medic/Firebat/Marauder and go hunting for the Zerg bases. However, it's more efficient to just run the SCVs to any high ground when the lava rises instead (It's best to keep them all in a control group to save yourself the trouble of selecting them in a pinch).

If you keep only 5 SCVs mining per Command Center, you can load them all into the Command Center when the lava rises and lift off. When you get the objective to go find the missing miners, do so, and you'll be granted a free Command Center and 5 SCVs. (If your playing on Brutal Difficulty, the timer will not be available, so you'll have to depend on the Adjutant warning you of the lava rise). You'll have plenty of warning and plenty of time to get your units to the high ground, so don't worry about that. Soon after the mission starts, the lava will rise, and you'll have to pull all your SCVs to the high ground. You start in the left middle portion of the map, with only a few structures and some SCVs. You can build Reapers starting from this mission.

Wings of liberty brutal