Go slow with your pup, first let him/her taste the toothpaste on your finger to get him/her use to it. Then try letting him/her lick it off of the toothbrush, after he/she is used to the toothpaste and brush you can start the actually brushing. The key is patience and persistence, that's how I'm able to brush Henri's teeth and he actually likes it!
I hope this helps!