Make your marines into one group and the SCVs into another. Take the marines to the southeast, following the road. At the end of the road head south to find Raynor in his vulture. Now add Raynor to the group and proceed to the southeast toward the base. Use the attack move order to do this. That is hit a then left click on a patch of ground. Your units will attack anything that stands in their way between their current location and their destination. You'll definitely encounter a small group of zerglings that will pose no threat. Raynor could kill everything on this map by himself almost. Keep the SCVs moving behind the units and you'll soon come to the base.

Set the five SCVs you started with to gathering minerals and build three more to gather minerals even faster. When you've got 50 minerals after that build one more SCV and set it to building a barracks. When that's completed start pumping out marines while the SCV builds a supply depot or two. This mission will be over with before you know it. If you're bored while your marines are building do some patrolling on the map and kill the zerg that are around.