There's no easy fix for that really. The only way for you to pull your grades up is if you really apply yourself from now on. Obviously I don't know your story or how much you put into your schooling, but if your grades aren't what you want them to be then the only real option is to study more, figure out how you best learn and use that to your advantage, practice, etc. As for the test-taking bit, that just comes with practice. Try working on public speaking or some other things of the sort. People crack on tests because there's more pressure, so if you practice doing things in high-pressure scenarios, it's easier to get used to it.
