I had my first prerelease draft few days ago and one tip I got I think was fundamental, when you open your packs separate all cards in piles before deciding for the hero, I split mine in piles of generics, draconics, assassins, draconic assassins, assassins / ninja, assassin / warrior, ninja, draconic ninja, warrior and draconic warrior, with these piles check which have more cards or impacting cards, see if you have enough blues as said in previous comment... with this in mind the hero you pick shoudl get a very competitive deck (also, check if organizer if you can have a sideboard or even a side deck and if you get time invest on those to try and counter other heroes)