Economic superiority is the single best advantage you can have over other players, especially if they're human.
At the very least, capture your 'half' (or equivalent, in the case of 3+ player matches) of the map as early as reasonably possible. Most maps have a vast amount of mass deposits available, and if you turtle up early and neglect expansion to these mass spots, you are giving your opponent a massive economical advantage, meaning more hostile units, more hostile upgrades, and more hostile expansion. Furthermore, you cannot harass your enemy effectively without, at least, a small assault force to attack them with, which brings us to our next point: Mobile combat units are vastly more useful than static defense buildings, and early-game ACUs are too slow to be used for expansion.
That said, there are specific guidelines that can make your life much easier when contemplating what to construct: Because of the flexible nature of RTS games, there is no single pattern to follow when building.
