When you sign up for service with T-Mobile you win a SIM card. If it's prepaid, that's included in the activation tackle. If it's not prepaid and you already have a phone, they'll probably only give you a SIM contained by the store.
However, if you're signing up with a contract picture, you can get a free low-end phone anyway so you might as economically take it purely to have as a backup phone, it never hurts. I penny-pinching, it's free, after all. You can see the current free phones at www.t-mobile.com/shop/phones.