Don't do your first ever fixed ride up a hill and then down. You will enjoy going up, and relax; and then turn round and start descending. You will then instinctively try to freewheel. It will not be a fun experience.
Chavs Local youth will overtake you in their car laughing at you. That night you will have nightmares and decide it was a really stupid idea. This will pass.
Some more tips for starting out:
1) Fit two brakes and make sure they work.
2) Learn to let your legs go gumby on the downhills (wot Clarion said)
3) Use clips, but make sure you can get out of them fast - at traffic lights you can't just freewheel with one foot ready to put down.
4) Seek enlightenment regularly from this very board and its resident gurus. The
fixed-gear mailing list is also a source of inspiration.
5) Stick at it, it's worth it!