First 5-10 minutes or so:
Start with a few mass extractors (should be at least 2 mass points near your start point), then 2-3 power generators. Stick a land factory down (try to attach it to the resource units, it will then take less resources to build units). Get an engineer out to expand and find more mass points, then some light units to harass enemy engineers. Put up some radar to help spot incoming enemies, then maybe a few T1 defenses if you expect an early attack.