I also have thick wavy hair that is a little on the coarse side, so let me share a few things that have helped wondrously with my frizz.
First off, I wouldn't recommend straightening it! I started straightening my hair when I was 13 (I'm almost 20 now) and if I hadn't fried my hair with extreme heat through my teen years, my hair would look a lot nicer now! As frustrating as it is to have frizzy hair, the dryness that heat styling causes is only going to make it worse. Not fun!
So that being said, here's my tips.
-Put product in while your hair is still soaking wet. A good frizz fighter for me is just a nice, simple gel with no silicones- I use LA Looks gel. A dime size amount for me is more than enough, any more and I get crunchy. The motion you use to put it in your hair should be not a finger-combing motion but more of a scrunch.
-Don't dry with a terry cloth towel, it makes your hair fuzzy and flattens your natural curl/wave patterns. An old t-shirt works very well.
-After you've put the product in, DO NOT TOUCH YOUR HAIR UNTIL IT'S DRY. That's probably the biggest thing that changed my hair completely, and you wouldn't even think it would make that much of a difference, but it does. The more you touch your hair, the more frizz you'll have, it's that simple.
-Try washing your hair every OTHER day instead of every day. If the hair's natural oils make their way down the shaft, it'll help hair be more moisturized and less frizzy.
I hope this helps, I know how much it sucks to be so frustrated with your hair as a preteen, when it seems to matter so much to have cute hair. Let us know how it goes!