The computer does actually have one unfair advantage. It starts off knowing where your base is, so it doesn’t have to go looking for you.
A few more points: In choke points, build Supply Depots and Bunkers, with the Depots in front. This channels enemies down even more and slows them. A single Bunker with two marines and an SCV repairing it is usually enough to hold off the first rush (unless the computer waits to get tanks). Also, only put Firebats in the bunkers if you’re up against Zerg or Protoss. Firebats are useless against Terrans. Don’t use infantry outside of bunkers unless you have Brood War and medics.
Another good starting map, by the way, is Station Unrest. It’s an 8-player map, so there are plenty of expansion bases, and every starting point starts off with an expansion base right in its backyard. Also plenty of chokepoints to hold off the enemy.
El Elvis Rojo, I disagree. In custom games the computer doesn’t cheat. It makes things faster because it doesn’t have to move a mouse around and it can do more than one thing at once, but its units build just as fast as yours. Use a cheat code sometime to watch and you’ll see.
Mereson makes important points about micro. Unit micro is especially crucial against the computer because it’s so dumb. Once you learn to dance well enough, you can start trying to win a game as toss without losing a single unit.
Baldwin, tanks are useless without siege mode.
My opinions on upgrades: buy them. They’re cheap (especially for Terran), and they make a huge difference. Infantry grades can win you the game, m&m (marine and medic) are a sick combo when the rines are fully graded (3/3). Cost for cost they own most single-unit late game strategies and any straight capital ship (carrier, BC, guard) producer. An engineering bay is only 125, each grade is 100/100. That’s just the cost of a few units.
Chronos is right, but his advice applies only to playing against computers. Against any decent player, a depot/bunker wall will only assure your death.