First build two supply pads, then build a barracks and then build another supply pad, then a reactor.

Wait until you have about 850, then buy the first upgrade for the Marines. Send 3-4 squads and have them and allies guard your base.

Upgrade your reactor, then buy the second marine upgrade.

Then buy the first two upgrades for the flamethrower and then build one more supply pad and an armory.

After that mass produce Marines!