Do you rinse your mouth with water after brushing? Some dentists say to not rinse so you keep more fluoride on your teeth to prevent cavities, but you need to use a really small amount of toothpaste, like the size of a pea for it to be practical. You should still spit out the foam and junk in your mouth when you are done, but don't swish with water after or it removes a lot of the benefit, especially if you don't brush for the full 2 minutes recommended. Wait 30 minutes before drinking anything after.
It is best to first brush the biting surfaces of your teeth (top and bottom), since they get cavities the most and need the toothpaste on the longest. Next, do the inside and then outside surfaces.
Adding water to the toothpaste is a good idea, since dry toothpaste would be too harsh on your teeth. The water also makes the toothpaste more active and works better to clean your teeth. Don't forget to brush your tongue too, or use a tongue cleaner for good breath :)