As a teen you don't want to look frumpy, but as a mom you don't want to look slutty! There is a comfort area, though
Make sure your belly is covered in all clothing, and that your neckline doesn't go down too low. I'd stay away from over trendy things, as well - they'll make you look younger, IMO, and they'll also be out of style very quickly, so it's no use spending your money on them when you probably need to spent it on other things - babies are expensive!
IMO you want to look classy. Which mean no bright blue hair, bra straps showing, combat boots, or mini skirts. I know that you might want to wear those things, or any other variety of those things, but because you're in a delicate situation I think you need to make sure people can trust and respect you, because now you're looking out for your daughter's well-being, not only your own.
I'd stay away from short skirts unless you're having a night out on the town with friends and the rest of you is covered. I'd say that means skirts to your knee or just a little bit higher - whatever looks best. I don't feel comfortable in anything higher than just at my kneecap, but my own mother wears her skirts a few inches shorter than that, because she has shorter legs and can get away with it. My skirts just make her legs look stumpy.
So...you don't have to dress like you're 50, but definitely don't dress like you're 13, either. You need to convey to everyone that you're a competant mother.
I have a friend whose daughter just had a baby. She is only eighteen. Having a child has matured her. Oh by the way congradulations and I will keep you in my prayers. Having a child is difficult at times but rewarding the same.
Back to subject. Whether your a mom or not you want to dress classy, even as a teenager. You can have fun though. Don't forget who you are. I imagine that you might feel like you will have to prove to people you can and will be a good mom. Just do the best you can and ask lots of questions from older women you trust and know!
I think vintage tee shirts and fitted jackets are fun with a great fitting pair of jeans. You can dress them up with a h*t pair of shoes or a pair of converse.
I just had a baby too. Your body just isn't the same anymore, at least for a while. This is my second child. I like to be trendy but age appropriate. Be comfortable but not frumpy. Find pants that work with your tummy. Please no low riders where your tummy hangs out and everyone gets to see your underwear when you bend over. That's not good even if you don't have kids. I found that I now need a higher waist but it still doesn't need to be above your belly button. Make sure your shirts fit over both your chest and stomach. No need to be too tight in one area because it fits in the other area. Good luck.