Now why would you want to make that beautiful head of curly hair straight? Goodness, I will never understand you young people. You should be so thankful you have a thick head of dark curly hair. Think of all the guys your age begging for a remedy to help them to stop losing their hair and facing baldness. OK, I know enough of the Grandma lecturing.
I'm not going to recommend any chemicals to put on your hair, even though there are many that will make your hair straight, including using a straight iron.
Try putting a thin coat of olive oil on your hair after you shampoo it, comb it through and let it dry and comb again. This should relax a lot of the curl, plus it it very good for your hair and scalp. If this isn't enough you could try heating some olive oil, just very lightly, and massage it into your hair and wrap it with a towel for twenty minutes. This will relax the curls even more.
In my opinion, leave your hair curly, you're a good looking boy and girls really like dark thick curly hair.