If you're coming from Mexico City, you can either rent a car and drive to Valle de Bravo (it's approximately a 2 hour drive) or take a bus in the Terminal de Autobuses de Observatorio, in the north-western side of town. There's only one bus company that takes you there but the buses and the service are acceptable. You can buy your tickets the same day you're traveling, there's no need to make reservations early (unless you're traveling during holidays or so). Most people that go to Valle de Bravo drive there themselves.
There are taxi cabs that can take you from one place to another within the town of Valle de Bravo or even to the surroundings (to the hotel where I stayed in Avándaro, for example). If you're going to stay within the downtown area the best you can do is just walk and enjoy the beautiful and rustic streets of Valle de Bravo. Only if you're going far or out of the town should you take a cab. You can take them on your own but this is more expensive: instead, you should take a "public taxi" which picks several people up at a time and follows a specific route, so that way you share the cab with other people that are going to the same place you're going and the price decreases considerably (even though the taxis there are fairly cheap anyway). They start working pretty early in the morning (6 AM or so) and you can easily get a taxi after midnight as well, even though by that time they're not "public" anymore because there aren't many people riding them...