If you have a medical condition, please get advice from your doctor before starting. You can hurt yourself if you do too much too soon. So please start slow. Start with 5 minutes of exercise and build up from there. Don't start with running if it's been years or even months since last you ran regularly. Start walking and gradually add short intervals of jogging until you are able to run the whole distance. This should take a few weeks. Don't rush it. If you strain and hurt yourself, it may be weeks till you can exercise again.
Rest between workouts, if you are really pained up, give yourself a day or two to catch yourself :-).
Stretch after your work outs.
Listen to your body. Pain isn’t always gain. Stop what you are doing if you have chest pain, severe shortness of breath, or sudden pain in a joint or muscle.
If you already have bad knees/ hips/ ankles, you should avoid high impact exercise like running, basketball, soccer, football, and some types of aerobics. Walking on grass, swimming, elliptical, bicycling are options for you to consider that should be easier on your joints.
Exercise is the magic pill that no one wants to take. You can make a huge difference for the rest of your life if you start exercising.
Love yourself, take care of your health!