lack of cardio exercise will cause you to have a higher heart rate.
I had surgery in grade 9 and was out of school and not doing any physical exercise for almost 5 months, and my RESTING heart rate at that time was around 100bpm. before then my resting heart rate was somewhere in the 60's.
As far as getting into better shape: weight lifting and shit, that hardly does anything for your cardiovascular system. things that get your heart pumping such as running, swimming, etc. will help you reduce your heart rate.
-katie