Aha last year was my first time going to a con... I felt the exact same way as you do currently
Anyway.... I took $100 in cash to Supanova (and spent every last cent), it should be enough to spend (I think....). Just be careful with it as you could easily loose it. Most of the stalls will probably ask for cash. I believe they have ATM's but the queues are looooooooooooooooooooooong so its always best to take cash anyway so you can spend most of the day feeling pretty awesome and so you dont spend half an hour in a queue waiting to get cash.
For backpacks? Well last year I didnt need one (since I'm supposedly like a camel, who can store water somehow) and all I carried with me was a wallet and my camera. But I suggest its a good idea to take water with you as it will hydrate you and you'll probably need it.
Just get out there, stay in character (or well try)... AND HAVE FUUUN