Geometry is a different beast from any other class you've taken at that point in your school career. People who normally pull Cs in math can pull As & visa versa (I watched it happen).
What do you mean by "not that great" in math, if you don't mind me asking?
One of the biggest tricks with making the transition between high school & college is being able to effectively self-study. In many college courses, the professor will lead the study & introduce the concept, but you will ultimately need to study on your own (beyond any assigned or suggested study questions) to get the depth of understanding required to do well in the course. (Don't worry, it's not as bad as it might sound, you'll get it when you get there.) The skill most people, including myself, constantly struggle to develop is studying in an efficient and effective way.
College classes will also have a quick pace. Try reading ahead to the next section the night before your class. This means the teacher should never blindside you with completely foreign information. You will also have the opportunity to ask questions about concepts you still don't quite get rather than not even knowing what questions you have until the next lesson & having to play catch up. I found it particularly important to use this tactic in math (especially my recent Elements of Statistics class!) because the concepts build on each other from one day to the next. It's cliche, but I really found that the effort I put into preparing for class dictated what I got out of it.
The most common pharmacy school math requirements are statistics & one semester of calculus. Some schools require 2 semesters of calculus. I'm not sure what the high school prereqs are for a 0-6 program are. I would suggest trying to take algebra 3/4 in high school so you're ready to enter college at least at the precalc or trigonometry level - it makes it a lot easier.
As for over the summer ideas... identify stuff you're weak on and work those concepts specifically. There are lots of great youtube videos & free websites to help you. If you run out of stuff, try going on yahoo answers & answering questions in the math section.
Fun fact: I never officially passed the 4th grade because I never finished math...
