This is the standard build order to use with Sergeant Forge when going for a mechanical army. This build can branch off to either ground based vehicles or aircraft which you will do depending on your opponent's build.
Start off building a supply pad and another warthog. Send your warthog to scout out the enemies base. Carefully micro your warthog to avoid being damaged by creep (non-player units including insurgents and flood). Use your warthog to see what your enemy is building and steal the supply crates near his base. Build another supply pad and use your second warthog to gather the supply crates near your own base and the middle of the map. Next build a reactor and a second reactor as you continue to grab crates and scout your opponent.
As soon as your second reactor finishes build either an airpad or a factory. Pick whichever counters your opponent the best. For instance; if your opponent is building hunters, build an airpad because air units are good against hunters.
As your airpad is building upgrade your base. If you have enough minerals you may even be able to start doing this before you start building your airpad or factory.
After your factory or airpad start pumping out units and fill out the rest of your slots with supply pads. At this point your opponent will probably be attacking you or expanding. You may need to build some turrets or even lock down your base for a while. After your army builds up to 4 or 5 units you should start expanding or attacking your opponent. Continue pumping out units and you may want to upgrade one of your reactors at this time.