Build a Barracks
Train 10 Marines
You begin the mission with 5 Marines and 5 SCVs. Take your troops across the bridge to the south-west and continue until you find Raynor. Head down to the south and take over the partially built base. Get your SCVs to harvest the minerals and when you have enough build a Barracks. You won’t need another Supply Depot unless you build more SCVs.
There’s a few Zerg around the map and you can send your troops out to hunt them down if you want. Once the Barracks is complete build another 5 Marines to complete the mission.
Next: Backwater Station