The first thing to building a dirtjump is the location. You have to build in a area big enuff to get alot of speed. Don't build jumps and trails on private property unless you have the owners permission.
  Now you are ready to build. You will need a shovel and water. If you are  just starting to dirtjump I wouldn't build the jump any higher than 2 or 3 feet. Once you get the feel for it just keep adding more dirt to the jump.
Shaping the jump is an important part of the jump. Shape the face of the jump like a 1/4th pipe. Make sure there are no flat spots and that the lip is strong and sturdy.You can either make your jumps tabletop or gapped. I  make my jumps gapped . I make the gap as wide as my jump is tall. The landing is up to you. I use almost twice as much dirt on the landing as I do on the jump.
Now that you are done with building and shaping the jump lightly spray it down with a hose, pack it, let it dry and then ride it! Always wear safty gear and always  ride with caution.