A few things mate:
* Train the legs and back just as much as the rest (ie chest, arms and shoulders) of the body.
* Don't ever go to total failure. Why? Because then your RECOVERY time will get extended.
I would suggest you to try the Max-OT routine which there is a link to up on top of this board.
Each and every workout during three months is layed out, and all you need to do, is basically follow it, and eat good, basic food. That means staying out of the fast food places, trying to start making pasta by yourself when coming home from school and shyte like that.
Most of us in here remember when starting out, how we all were very ambitous, almost afraid of not training enough. Avoid the injuries we've collected, train duly. When you train, be 100% focused, don't jerk around in the gym. But don't overtrain. That will get you injured.
Hell, you'll probably ignore most of the advice, and get some injuries. But at least then you will know what to do. I didn't have much of a clue during my first 2 years training, despite being plagued by minor injuries the whole time...
Good luck mang