Hints for you: Making a decent 60-card deck out of 200 random cards is difficult, let alone 84 (6 packs of 15 cards, -6 basic lands)
Keys to success at the Prerelease (or really any Sealed Deck event):
1) Set out all your cards sorted by color (with artifacts separate). Then, sort each color first by creatures vs. non-creatures, then by converted mana cost. This will help you look at your colors to decide which one you want to play.
2) Also take note of any of your really powerful Mythic Rares (these have an orange rarity symbol), Rares, and Uncommons. And Commons, too, but most Commons aren't really powerful unless something is wrong with the format. You'll know if you have a really powerful rare. You want to try really really hard to play as many such powerful cards as possible in your deck.
3) Try to play 2 colors. Other people may suggest you play more or less, but if you're just learning, stick to 2 colors. It will probably work out better for you as a learning point.
4) Don't play more than 40 cards. It's almost never worth it.
5) DON'T put the prerelease foil card into your deck. The judges and people running the event don't like when you do this, and they will tell you so. So don't do it.