I have just been diagnosed with fibromyalgia and been told I must exercise. How much exercise should I do?

More important than how much is "what form?" and "how hard?".

What form?
Fluid exercise, such as swimming and low impact exercise such as Walking, Pilates, yoga or Tai Chi are best. Cycling is a fluid exercise but doesn't involve full-range limb extension so doesn't aid flexibility.

How hard?
Enough to raise the spirits, little enough to cause additional fatigue. Exercise raises mood-lifting serotonin and pain-killing endorphins, hence it's importance in fibromyalgia.

If you are starting afresh, I would suggest 30 minutes 3 times a week. Keep it aerobic in nature, so that you could hold a conversation while exercising.

I hope this helps

Best wishes

Earle