Congrats on trying out your first 5K, it's a very motivating experience to finish one, regardless of what time/rank you come in at (I did my first mid-Feb, and now am training for a 10K)
I don't know about you, but I'm a fairly heavy on the sweating, so I always lug a bottle of water (~24 oz.) with me. Yes there are water station(s), but doesn't cut it to keep me hydrated (or I'll have to stand there to drink a dozen paper cups to re-quench myself

).
Also, if you have a light music player, bring it along and play some upbeat tracks to keep you going.
Also, do some warmups before the run/walk to get your muscles geared up, and some cooldowns to avoid cramps and reduce soreness. Also, dive into the fresh fruits they have after the run -- you need that carb refuel! (Just don't go crazy on the energy bar samples some vendors may offer, tends to be higher in refined sugars.)
Finally, don't over exert yourself, if you have to walk - walk. Better to make it a few minutes later than expected than to be dragged in by the pickup or the ambulance.
And of course, HAVE FUN!