How to activate tmobile SIM card for iPhone?

I upgraded to a Tmobile iPhone 5s that came in the mail last night. It came with a new, small SIM card in the slot, so I'm using it since my old one's too big. How do I activate it without going to the store?

You could do it online @ (https://www.t-mobile.com/shop/plans/ActivateCodes.aspx) or call the T-mobile activation center @ 1-877-453-1304 if only they are open today.