Assuming that you have maintained grades in that range prior to high school, I suggest doing the same things you've been doing. High school is radically different in many ways from previous grades, but the components to academic success are unchanged:
1) Maintain good attendance. It's hard to keep up in a class when you are doing, in essence, twice the amount of work at one time, which is what you have to do after an absence.
2) Stay organized. One of the biggest challenges for students of any age seems to be finding an efficient method of organization. This eliminates time wasted looking for the paper that you have to have RIGHT NOW...
3) Communicate, communicate, communicate! If you have given something your genuine, best effort but are still having trouble with it, let your teacher know! As much as we would like to be, we are not mind readers, and if you don't ask for help, we sometimes don't know that you need it.
4) Relax. Stress makes it easier for you to get sick and harder for you to keep up. Make sure that you aren't overcommitted. In high school, there are often conflicting obligations and committments that include academics, sports, clubs and organizations, work, church, and family. You have to learn to prioritize.