Before exercise, you should generally do a warm up that mimics what you're about to do, just at a lower intensity. For example if you're planning a run, start with a slow walk then build up to a jog, or if you're playing netball, go through the motions of the movements you'll be using etc.
Combine these with dynamic stretches. These involve muscle movements that move a joint through the full range of movement required in your chosen sport or activity. Think leg swings, walking lunges and squats.