1) Go onto the Horizon web site and read some of the articles that talk about 3D.
2) Buy a Sim and practice 3D
3) Get a plane that can 3D that has lots of power. I learned on the Extreme 330 w/ a Saito 72.
4) Don't be afraid to try new things and potentially crash
5) Start high

Basic 3D is easy - just throw the sticks and it's done. After that you need to learn how to control the maneuver.