What you wear is up to how well you tolerate temps and what the weather is on race day. The rule is to wear one layer less than you think you'll need because you'll warm up during the run. Wear layers that you can take off if you get too warm. And you can always bring a trash bag...you can wear it while you're waiting for the race for warmth then toss it before the race starts. If you don't need it, use it to sit on.
Good luck on your 5K! You'll do great! Be sure to post a race report when you get a chance!