Start out with a tabletop because they can less intimidating. Find some dirt and start shoveling. Make a mound of dirt, shape the lip, shape the landing, pack it with your feet, hit it. Thats basically all I did until I figured out tractors work too.
you definately want some fill (rocks, logs, old tires, whatever is close by). it is good to use plywood for the jump as well. it doesnt wear away, and it holds it shape through rain. it helps if you have a lot of junk laying around to utilize. just use your imagination and be ready to work for a couple of hours. start by hitting it small, and then build your speed when you feel comfortable. before you know it, you will be covering 15 horizontal feet. make sure your lip is smooth, or your back tire bounce off the lip, hit your ass and drive your nose into the ground. also, the key to a good jump is the transition (the part where the ground transitions into the jump). it cant be too steep or drastic, or it will throw off your approach. remember, hitting a jump on anything (snowboard, skateboard, moto, bike) is all about being stable when approaching the jump. you better have control of your ride before you hit anything, and hit it straight, or you will probably find yourself in an unwanted tweak. have fun, start small, stable jump, stable approach, good transition, and good lip. that all results in good, solid, comfortable air.