Received Birthday Invitations

My colleague just gave me his daughter birthday invitations card. I am now headache on what kind of birthday gift I should buy for her. I have no idea what to buy for a 12 years old girl at all. How about clothing? However, I didn't know her size, then how do I choose the cloth for her? This not a good idea, I might buy the wrong size. Maybe I should consult his father what kind of stuff her like. I just want to get her something that she really likes.