I'm not entirely sure how it works, but it says I'd get Unlimited Data, Voice, and Text for 90 days, I believe. I know I probably need to set up an account. Does it come with a SIM card for the phone? Is it like the other cards; does it charge per minute/text/kb? Does it auto-renew, or do I have to buy a new card everytime?

I just want some general information before comming some money to it.

More info: I plan on getting the AT&T Avail, with the $50 Pre-paid card. 

Edit: Can I link the go phone to an already existing account? If so, is there a charge?