If you use basic desensitization methods things tend to go well. A toothbrush, nail clipper, dental scaler, etc are all weird new experienes for a dog, so it pays to work up slowly.
A basic quick overview of desensitization is to use food/treats and offer the reward at the same time as you introduce the toothbrush (clipper, etc). Work in small increments, first just go for touching the lip, once they're OK with the brush touching their lip, distract with another reward and touch their lip for a bit longer. Build up step by step, making it a pleasant experience every step along the way. If you find the dog is hessitant, go back a few steps and work forward slower. It can take more time at the begining, but it pays off in the long run. As you progress you get to the point where they will sit there, often they will lift their lip when the brush comes in, give a good brushing and offer the reward at the end. There are tons of resources out there on desensitization training, watch a few videos and do a bit of reading, it's an investment of time that will save so much frustration for you both as time goes on.
I trim nails with a rotary sander and the dogs are fine with it; I brush teeth, use a dental scaler, etc. It's the same process of bulding up trust and can be used to get the dog to offer a paw for even blood draws and other procedures that are typically more traumatic but don't need to be.