how do you repair a dent in your surfboard using suplies avalible at surfshops?

Do you know how to work with Fiberglas?

If you don't, let someone else do the repair.

Is it just a dent or is it cracked?

If it's a dent don't worry about it, you're going to get more, trust me. If it's cracked take it to the shop or ask the shop who does repairs. Fiberglas and resin can be very messy and you don't have much time to work with it before it starts to harden. All beginners put to much hardener in the resin when they first start and it hardens faster than they can work with. That's funny, they have resin everywhere and fingerprints all over the board. I just laugh at them, they screw up the whole thing. Learn to work with Fiberglas and resin first before you go screwing up your board, I just might see you and laugh at you too!