Why does my hair always frizz up?

I'm 14 years old and I've been dealing with frizzy hair since I was 10. I just started straightening my hair a year ago and realized the frizz has been going down. all of a sudden, I straighten my hair and it's pin straight. then I go outside or I go in school and BOOM, my hair gets frizzy and curly. how can I get my hair to stay straight and less frizzy? (I use heat protectant)