I'm 21, see over here <-- to see where I began, and where I'm at now, and where I want to be. Maybe it's just because I'm 4'9'', but after the first 10lbs I haven't seen much physical change in my stomach and that worries me. Maybe it's just because I'm 4'9'' and it really has nowhere to redistribute to, but I'm getting scared. I don't want saggy skin. A little sag, fine, but I don't want a ton of it.

So I have to build muscle there, right? How do you do that, again? sit ups? I just don't wanna be saggy

There is not much you can do about saggy skin, you'll either have it or you won't. There are many many factor's that play into that - genetics, age (you're young that will help), how fast you gained, how fast you lost, etc. etc. There are a couple stickied threads around about the topic as well. There are also some lotions and creams, but most of those provide little results.

Building muscle will help, but you can't spot reduce and you can't sit up away saggy skin.

I wish I had better news for you. Don't worry about not seeing many results after only 10 lbs, that's normal.

For what it's worth, I've lost 120+ lbs and I'm 31 and my saggy skin isn't that bad. It's certainly coverable with cloths and I can wear tight fitting clothes without a problem. Fingers crossed for you.