I felt like I didn’t understand anything in calc AB and I was a senior lol. For the test prep, scour Google for cram sheets, do Khan Academy, just tackle one theorem at a time. As for your grade… my calc class didn’t have any assignments after the test so ngl you might be cooked 
But it’s not the end of your college hopes. Since you’re a junior, worst case scenario you retake the class… or reconsider your intended major… it doesn’t completely eliminate your dream school, I didn’t even have my test score or calc grade when I applied and I still got a few acceptances to somewhat reputable universities lol. You’ve gotten some good advice on this forum, but we don’t know all your circumstances. So definitely talk to your teacher or guidance counselor and see what you can do going forward. Good luck 