A while ago I did a video for TW Osteopaths, showing how to do a general warm up. A warm up should be split into two parts, general (for all activities) and specific (geared towards the activity you are about to start). No matter what sport you are going to do, be it football, swimming, gym workout, dance, you should do a general warm up. Once your joints and muscles are primed then you move on to more specific things for your sport.
The basic idea is that you want to mobilise the joints, increase blood flow to the muscles, activate the muscles and elevate the body temperature, heart and lung capacity.
Check out the video on Youtube and get in touch on here if you have any questions.