if its unhealthy hair you should get a haircut and start over. if its "your hair" there are lots of smoothing products and tools to straighten, smooth and shine it if u want to wear it down. braids, twists, buns and up dos are endless for that length if you like working with it. keep it healthy. I got mine cut into layers from that length and I'm loving it. its so much easier
If you will buy a flat iron, you won't have frizzy hair. Take a small section of hair and iron it from top to bottom. It takes all the frizz out. When you can afford it, buy a professional model. I have a professional Chi and it leaves my hair silky and straight. I think you will really like it.