First, start with those normal stuff of building your scv's, supply depots and whatever. Build two or more barracks to build your marines.

Next, when you think you have about 12 or more marines, you start building about 8 firebats. Build about four bunkers around/near a choke point and sort of clog it up.

Fill the two bunkers nearest the choke point with firebats and the remaining bunkers with marines or ghosts. (If using ghosts try to get the ocular implants upgrade)

If you think that your enemy is going to use melee attackers, just let the bunkers stay that way. But if you know your attackers are using missle or whatever long ranged attacks, switch your firebats in the bunkers with marines.

Be sure to keep around 2 scv's near the bunkers to repair damaged bunkers.

CreepColony's Opinion: I personally think that bunkers at your choke point are only good against comps. Humans will usually go mass ranged ground (hydra, goon, etc) or mass air. If they go mass ground, then put tons of tanks at your choke point behind supply depots.