Should I trade in my iPhone 4S for an iphone 5?

1. If I have tmobile for my carrier do I have to trade it in with tmobile?
2. If I trade it in with tmobile can I ever change carriers?
3. If I have an iPhone 4S in good condition how much will it cost to trade it in?

Answer these questions in that order.

Don't ever trade stuff in to stores, they just rip you off, refurbish it, and sell it for more. If I were you sell it on ebay/craigs.