Process to port number from AT&T to T-mobile?

Hi,

I have a AT&T connection and I would like to port it to T-Mobile without changing my number. Could anyone let me know the process to do it?

I went to local T-Mobile store but they cannot provide me the plan ($30 UL Web and Text (5 GB at 4G) ) from the store. If I want this plan, I have to do it online. Weird. So can anyone help me out here? Thanks.