Starting Point – Your base has a good supply of mineral and Vespene gas. Expand your Marine forces to 12 soldiers and head North. Raynor can set up mines around the base to help secure it.
Zerg Blockade – A group of Zerglings and their structure are blocking your way to the isolated Terran facility. Your Marines will easily deal with this threat.
Isolated Terran facility – Once your units come close enough, the facility will be placed under your control. The site has additional resources and production buildings. This should be your new rally point when you're going to attack the Zerg.
A arge group of Zerglings – The spot hosts a Creep Colony and a full squad of Zerglings. A big enough group of Marines should deal with them without any problems.
Zerg defense line – You'll have to destroy all Zerg structures and eliminate all enemy units in this area. The enemy has gathered a significant number of units to make sure you have at least two full squads of soldiers.
Infested Command Center – While in theory it's a Terran structure, it's been overrun by the Zerg and needs to be destroyed to complete the mission.
Groups of Zerg – Small Zerg formations are scattered across the entire map. Some of them have buried themselves in the ground, and will attack if your men come close enough.
Mineral Deposits – If the southern and northern bases run out of resources, you can either build a third base here or move one of the other two, e.g. the one from the starting point. These extra resources are protected by small Zerg forces.