1. You must purchase an AT&T sim from ST website and a $45 month card either at the website or at Walmart.com or your local Walmart.
2. You will have to decide whether you are going to port your number to ST or get a new one from the, or port your number to Google Voice and forward it to a new ST number. If you are going to port, you'll want to do that when you activate the sim and start your service.
3. As soon as your port is complete (or as soon as you finish the activation process if you aren't porting), then you will have voice and texting working. You will have to change some settings (seen here on the forums as APN settings) to get your data and MMS (picture messaging) working. There are a couple threads here that tells how to do that, just come back when you have your sim and are ready to switch.
4. Purchase your monthly card every 30 days, and you'll have virtually the same service that you have now with AT&T, just for $45 a month plus tax. You do need to be aware of the "safe data limits", which are 100MB per day and 2GB per month. You should install a program like 3G Watchdog to help you keep track of your usage.
Any more questions, just ask. We're generally helpful here, just gets frustrating when the same question has been answered multiple times in a week.