What do I do if my dogs heart is beating very fast?

Put the dog in the crate (or where he feels the safest) turn the lights off, don't bother him and call the vet Immediately!! This is not normal nor is it good...

I agree with Lucky.

There are many things that could cause an elevated heart rate.
While it may not do any damage short term the long term effects can be very, very bad.

It's kind of a vague question, was the dog playing/exercising before? If so it will go down by itself.
Also I notice my dogs heart beats fast when he first wakes up, the vet said that was fine he was just exited to be awake. If your dog was just laying there and his heart started to beat fast take him to the vet.
--A dog's heart beat can vary from 100 to 150 beats per minute depending on the breed. Normally, the smaller the dog the faster the beats.