To roll down a ramp you will need balance. It will really open up skateparks to your skating.
To roll down a ramp you will want to be able to roll and kickturn at a pretty good speed. You need to know how to shift your weight around on the board while rolling. Shifting to adjust your feet and being able to control you balance. You need to be able to crouch and handle a good amount of speed.
When you skate down a ramp you will pick up a lot of speed. You need to be able to relax, stay low and ride it out. At first you may tense up when you start rolling faster. Try to stay loose. It will become second nature soon.
Find a ramp that you can roll down. It can be at a skateparks or someplace in the streets. It needs to be flat on the top and the bottom. You will need enough room to start rolling at it at a 90° angle. You will be rolling straight down the ramp.
Start pushing and get enough speed.
Just do this a lot. Get used to the speed. Get comfortable with the crouched loose position. This will be useful in keeping you on your feet. As you get better at skateboarding you will need to crouch less and get more upright and relaxed looking.