I usually will kneel on the floor when I brush Tilda's teeth and simply put her between my legs. This way I just have to lean over a little, open her mouth and start brushing. It does not need to be done this way. I can imagine this may not really work for a big dog. This is just one way to do teeth brushing.
Don't use your own tooth paste, it isn't good for dogs. I recently got Tilda a new tube of doggie tooth paste, so far I like it. It's made by NylaBone and is made in the USA. It's also natural peanut flavored. The tooth paste I used before I got this was made in China, something I wasn't crazy about.
It is best to brush everyday if you can. If you have a puppy, go ahead and start. If you have an older dog, go ahead and start. You may have to build your dog up to doing this, you might not be able to simply get in the mouth and brush. There is nothing wrong with taking time to teach this to your dog. Maybe you'll only be able to touch the brush to the teeth and that's all the dog will take. Take it slow and practice everyday getting more advanced in the brushing as your dog relaxes. Just because your dog doesn't like it, doesn't mean you should give up. Any brushing you can do will be better than not doing anything at all.