Purchased a Lumia 900 off contract, now how do I get service?

I just purchased a Lumia 900 today from an AT&T authorized re seller for the off contract price. I tried to get a no commitment plan set up in the store (preferably pre-paid) but the sale rep (and her manager) refused to do it without me signing a full 2 year contract, for roughly $100 a month. Also they stated that if I cancelled at any time I would have to pay the full early termination fee. This was shocking to me as I had just purchased the phone in cash for $490 with tax (they wouldn't even accept my debit card, btw just cash.) I was furious, so I left the store without setting anything up, and tried to do it online on the AT&T website. I ended up talking to a customer service rep via their chat program, and he stated that the store was wrong, and that I should get a Nation 450 plan. plus 3gb of data. I asked him several times if you could do this with the Lumia 900 and any smartphone, and he said yes profusely (I have the chat logged saved.) Being a second phone this basic plan is all I need, so I called up the store and explained what the online rep told me. The store rep got mad at me, refused to listen, and hung up. I double checked with the online rep again and he confirmed that the plan is available, but I have to get it through the store not online as they don't offer the mini 4g sim card yet. The store sales rep are terrible, they didn't have the phone out, and had no idea what it was when I tried talking to them. The woman tried selling me a Titan II instead of the lumia, and I had to get the manager just to get the phone. Anyway, can someone please let me know the correct answer? I really want to get this up and running as soon as possible. Thanks