Aerobic Exercise : Running, jogging, and biking are some examples. You're moving fast enough to raise your heart and breathe harder, but you should still be able to talk to someone while you're doing it. Otherwise, you are pushing too hard. If you have joint problems, choose a low-impact activity, like swimming or hiking.
Stretching : You'll become more flexible if you do this a couple of times a week. Stretch after you've warmed up or finished exercising. Stretch gently -- it shouldn't hurt.
Strength training : You can use weights, resistance bands, or your own body weight (yoga, for instance) for this. Do it 2-3 times a week. Let your muscles recover for a day between sessions.