Terrans are a very good race if you know how to play good with them. Your every multiplayer game should start like this:
1. Build an scv with 50 minerals you have,
2. Select your scvs and sending them to separate mineral patches,
4. Continue building scvs up until you have reached 8,
5. When the 8th scv pops out use it to build barracks,
6. Make 9th scv and use it to build a supply depot,
7. Build 10th scv and another supply with it.
It is very important to remmember exactly this initiall build order, because all the others are based on it.
If you are playing against Zerg, you will probably be zergling rushed. On most of the maps you will be attacked in 3 minutes, meaning that you won't have much marines. In this situations use your scvs to defend: retreat your marine(s) among the scvs and order 6 of your scvs to attack the zerglings while the marines hit some extra shoots. You will loose like 1 or 2 scvs if everything goes good, but if zerglings begin to retreat, don't follow them, but construct a bunker, 2nd barracks and attack him if you haven't seen any creep colonies when you scouted him. If he did get any, don't get 2nd barracks but a refinery and go for siege tanks.
If you are certain that you are going to be rushed don't build 2nd supply, but instead a bunker. If you do this fast you'll have about 2-3 marines in bunker what should be enough to defend it.
Zealot rush is a powerful against Terrans, just because most of the players don't build any firebats. After the initiall build order always keep producing marines.
1. Do the Inital build order,
2. When you get enough scvs (3 on 2 minerals + 2 for constuction) build 3rd supply and a bunker,
3. After 3rd supply is finished build refinery and 2 scvs,
4. After the bunker is finished build academy,
5. When the refinery finishes, put 3 scvs on it,
6. Start craniking firebats as soon as your academy finishes,
7. Build 2 more scvs which will build another bunker and the 2nd barracks.
If done properly, this will pretty much kill off any zealot rush, and if you are zealot rushed, after you kill it go and kill his base. If your scouting shows that your enemy is going for the dark templars, just construct a comsat and/or engineering bay. Otherwise, get tanks.
If you are playing game against Terran (can be done aginst Zerg and Protoss as well), and you are Terran yourself, the key to victory would be to prevent your enemy from expanding. This is sometimes hard, but for an early tactic I recommend the following (presumably you have choose a map with ramps):
1. Do the initiall build order and with the scouting scv build a bunker just at your enemy's ramp on the higher grounds. If he had already began doing this, construct the bunker at the lower grounds, near his ramp, but out of sight,
2. Rally your marines to the constructing bunker,
3. Build another offensive bunker and 3rd supply at your homebase,
4. Build scvs to fit (3 -2), construct 2nd barracks and 4th supply,
5. After you have filled your 2 bunkers build an expansion, refinery and 5th supply,
6. As soon as you have enough gas build a factory and the machine shop,
7. Upgrade tank siege mode and start building tanks,
8. Rally tanks to offensive bunkers,
9. Research spider mines, build 2 vultures,
10. When the vultures come and the spider mines finish, start putting them near your enemy's base.
If he haven't noticed you untill this he probably will now. At this point you'll have advantage of having two bases. I really dont't know what makes almost all the Terran players so afraid of expanding, so this tactic will work in like 90% cases, unless they spot you by luck. As you might have noticed this one is really complicated, so I suggest you to practice it against computer until you master every move.