How long do you have the dog / how old is the dog?
Did it happen before?
Did your dog eat?
He/ she can be stressed or cold, this is innocent and you don't have to do anything. You can put in a warm water bottle, but how warm is your room?
But there can also be a problem with the glucose level in the blood. Low glucose levels cause trembling.
This can become a serious problem, so if the trembling doesn't stop after 1 hour, please give him/her sugar water to drink. Best is grape sugar (don't know if that's the correct english name)
If your dog keeps on trembling, or trembling getting worse, please contact a vet.