Should I start exercising?

Well I'm almost 17. I'm pretty skinny. I'm 5'5 and weigh 105. I don't want to exercise to get muscles I'm afraid I might be getting big. My butt is I guess normal for a skinny guy. It's kinda small but like it jiggles like fat. Ew. Well you know how a but has that one crease at the bottom? Well. It looks like mines getting another crease below it. It's not like just a crease but it goes out then comes back in a little.