How to study for finals? help!(read description)?

Man I've gotten my self into some deep trouble.I promised myself to do good freshman year but i didn't. out of these four quarters I haven't done good in any classes, except french and guitar, which aren't as important. My other classes will probably average out to C's for the final grade.and now I'm worried because finals is worth 10% of our grade. and if I don't pass with a 70% for the whole year of that class I will have to retake it next year. Tomorrow I'm taking my science finals.And Im horrible at it. first I need to know How do I calculate what I need on the finals to get a 70 or higher. and second is studying now(cramming) a bad thing for me to do? and if I do cram will it at least give me a good grade?