whats better att, tmobile or metro pcs?

Go to local phone shop and tell them what you want.They will help you.
Before use your new sim card,Make sure your phone is free from any network lock.If it's locked by particular network,you will need to unlock your phone.For unlocking you can approach your service provider or else try to buy an unlocking service from any one of the online unlocking service provider like UnBBLock.com They will provide an unlocking service for Blackberry mobile models only.If you have a other than Blackberry phone means use Unlock-Zone.com