http://prepaid-phones.t-mobile.com/prepaid-plans
Scroll alllll the way down to the bottom, look for the $30/month plans. They include 5 GB of data per month at 4G speeds, after that you're throttled. I've never come anywhere near to hitting that limit, so I don't know what the throttle is.
You can order a SIM card online, or pick one up in a store.
http://prepaid-phones.t-mobile.com/phone-sim-card
Once the SIM is in your hand, then you can go into one of their stores or online to get one of the $30/month options.