First you need to find the cause of your grades. Are you not applying yourself? Do you not study or do homework? Do you fool around during class so you're clueless when test day comes? Or do you really try and truly not understand the curriculum? There are a ton of possibilities as to why you're failing. And before you find out how to fix your grades you need to find out what caused them in the first place.
I totally get what your saying! I get bad grades in one of my classes too! Even as hard as I try to study I just can't seem to pull a good grade! So just try to keep studying as hard as you possibly can. But if you can't seem to pull good grades, try to not stress to much about it. Just keep doing your best and that is all your teachers ask for.