You should have filled out a W-4 when you went to work to set up your withholding amount. If you haven't got a job yet, you will. My advice, always choose the Single withholding rate (even if you are married with kids). I have a little extra held as well, to make sure I don't have to pay (I hope that extra $K covers it this year).
Anyway, your employer will give/send you your W-4's by the end of January. The law says they must be postmarked by midnight on the 31st. Once you get them (from all the jobs you had during the year), you use the info on them to fill out your taxes. It's not as hard as many make it sound, as the forms tell you what box to look in and such. Doing it online is even easier, and the programs are the same ones used by the tax places.
As long as what you've paid in is more than the adjusted taxes on what you made, you will get a refund from the IRS. If you paid in less, you will have to pay the IRS the difference.