I ran my first 5K two weeks ago, during week 7 of the 9-week "couch to 5K" program (yep, I was jumping the gun). I found I started out a little too fast, and ended up walking for a few seconds near the end. I ran the same race again last week, and paced myself much better. Improved my time by about 40 seconds, but more importantly, felt much better. A lot less winded, and no cramps. Yay!
So my advice, from one newbie to another, is pace yourself comfortably. I like to have a little extra to turn on in the end, rather than use it all up in the beginning.
Oh, and good shoes are good! I got a new pair between races and I think they might have helped.