Keep it safe
Should I see a doctor first?
If you're perfectly healthy, it's up to you. If there's anything you're worried about - perhaps you have a specific condition or want to check about the particular activity you're interested in - then do. Don't worry about bothering them; they'll be delighted to see someone who's trying to improve their fitness.
What if I feel unwell, sore or exhausted?
Stop exercising if you feel unwell or if anything causes you pain. Talk to the instructor and see your doctor. Remember not to do heavy exercise after a big meal.
If you haven't done much exercise before, you may feel a bit sore and tired when you start. Listen to your body. Avoid sudden or jerky movements, and if it seems more than natural stiffness, ask for advice. It almost goes without saying that if you have an infection or illness, even if it's just a cold, take it easy until you've recovered.