I started by getting Kili used to having her teeth looked at while she still had her baby teeth. I didn't worry about rushing to brush those. Those come out. Do the ground work first. Every day, several times a day, get your puppy used to having her teeth looked at. Just look at them. Once she's pretty cool with that you can start touching teeth or gums gently just with your finger to get her used to something being in her mouth. Make sure when you look at the teeth that you get her used to having her lip pulled right back to expose those molars. If you do this every day then by the time your pup is 6 months old and has all her adult teeth she could care less about her teeth being looked at.
Now you bring out the toothbrush and toothpaste. If your pup is anything like Kili your challenge won't be introducing the toothbrush into her mouth, it'll be trying to stop her from swallowing the darn thing in an attempt to eat the tooth paste. Take it easy with the brushing at first. Just get her used to having the brush in there. Don't worry if you don't brush every tooth or don't brush for very long at first. Just make it a good experience.
If you do your job right you'll have a puppy who really wants her teeth to be brushed. Kili would actually cry in the beginning when I was brushing Summit's teeth because she couldn't wait for me to do hers. She's since learned that crying results in waiting longer. ;)