The $50 a month on AT&T is for unlimited talk, text and web on a Basic or quick messaging device (Non-Android) The avail, being android, is only eligible for the Talk and text for $50 then if you want the android data they have tiered plan for different amounts, something like 200MB, 500MB 1GB ect These all cost extra on top of the original $50.
No credit card is needed, you just get the phone, in store or off the web, it usually comes with a SIM. You Activate it with a number ported from your old provider or just get a new one issued to you, pay for your plan with cash (In store) or credit/debit over the phone with 611.
Once you pay the $50 it is good for 30 days from that day. just add another $50 in a month or let it expire if your done with the service and decide to move on.