Make sure you've got 3 SCVs on gas as you'll need 100 gas for the Factory once the Barracks are done
. Build a Starport as soon as the Factory is set and build the tech lab while that's being built. Extra minerals should be used for the Orbital Command. Once the Starport has begun you should also build a Supply Depot. Lift up your Factory and Starport and swap the two so the Starport can use the Tech Lab, saving you time rather than building another Lab. Once the first Banshee is completely set the rally point at your opponent's mineral line. You can easily harass their minerals early on this way before your opponent will likely have any anti-air to throw at you. To counter this you simply have to scout as again this does take 6 minutes so that you can plan for anti-air accordingly. Scouting is essential for reacting to whatever tactics your opponent goes with, so use it as often as possible.