How often should i exercise and what to do to make sure i don't injure myself?

First of all have a: good stretch. Your legs, torso, arms. Exercising at least 3 times a week for 30 minutes at each time is the average. You could increase the duration or the frequency. You much however allow time for you body to rest in between.

Every day: You should try and exercise every day. To prevent injury get advice from a knowledgable friend or even better a coach or trainer. In general take it slow, use proper technique for whichever form of exercise you choose, and focus on whole body work outs. Good decision to get more fit.