First, you want to gain some confidence and skill by dropping off of smaller ramps. Another great place to learn to drop in is at a skate park. Read How to Drop In off a Ramp or at the Skatepark, and find a small quarter pipe, mini-ramp, a small skatepark wall - something like that to practice on. It's good at first to find a ramp that has a long clear flat area in front of it, that you can roll out your speed on.
Once you feel confident on smaller things, you will be ready to work your way up to the bigger stuff. Try dropping in off of larger ramps, find the largest drop at your skatepark, things like that. Take your time with it though - relax, and don't rush it. Build your confidence at your own pace, and make sure you're having fun!
Once you feel confident enough, you can tackle dropping in on the actual vert ramp...
