Definitely go terran, as ottenni says, they are easiest to pick up, and can produce a reasonably strong marine force quickly and without too much difficulty.
The best strategy tip in general though, is to keep your enemy scouted at all times if you can. Run an SCV through at the start to find out what's going on there. Do some quick googling on the available units, their strengths and weaknesses. Another useful strategy would be, when you have a few units, to place on in each 'expansion' area, so you can see where your opponent is expanding too (which will help you determine attack targets).
Be flexable and fluid with your units, if you create a blob of a single type, and get spotted, the chances are that blob is going to be destroyed easily (You'll run into a lot of siege tanks if you have a blob of marines, for example).
Expand only when you are comfortable, as terran build a new command center in your main base, and float it out, rather than building at the new point, this will give you some additional safety, but make sure you put a few units near and around it, so you can spot an incoming attack (a few transports might be useful to get units into a defensive position faster).
Be as relentless as possible, try to avoid giving an enemy room to breath, once you have a small force, go in for the attack, they may loose, but you've gained some intel into what the enemy is doing. If you turtle, block and sit back in your base, all you're doing is allowing the enemy to expand at will and dominate you on economy, you may wipe their army out, but they'll be able to rebuild quickly! *ALWAYS* leave some units back at your base, and make sure you are producing replacements when you go for the attack, the potential of a counter-attack is large, and you need to make sure you still have defence.
It's gonna be a pretty big learning experiance for you, but if you watch how others play, you'll get a much better idea of what you need to be doing. Expect to lose a lot (It's unfortunately the way these things work), get moral from the little victories in-game
Good luck, and have fun!
[EDIT: this may have been a little more than you needed

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