I was given a 2G iPhone as a gift. How do I get a plan?

I tried plugging it into my computer, but I don’t have an AT&T number so I could not proceed on iTunes. I called AT&T and they told me I needed to go to a corporate AT&T store, but that is after I had been to a corporate AT&T store and they told me to sign up online. I am going to go back to the store, but any other ideas???