Depending on the severity of tartar & calculus you may want to inquire about having a dental cleaning done, that way you can start off fresh.
Getting your kitty used to teethbrushing takes time, it definitely isn't something that happens overnight.
First you need to get him used to your hands around his mouth by rubbing his cheeks. Then once he is accustomed to that, try some tuna juice or something similar and let him lick that off of your finger, from there try rubbing your finger on his teeth. Then once he's used to that, try pet toothpaste in the same manner as the tuna juice. Finally, introduce a toothbrush (most cat owners that I talk to prefer gauze, fingerbrushes, to electric toothbrushes). After every session reward him with a yummy treat! The goal is to make it a positive experience, so as soon as he gets fussy, let him go and try another time.
The best time to try this is when he is relaxed, and do it somewhere comfortable for both of you.
BTW, you don't have to open his mouth to do this, you only have to brush the outside of his teeth.
Hope this helps & how wonderful that you want to start brushing!!!!