hey mate, everything you need to know about building dirt jumps is
here. The farkin forums have search tool, learn to use it or you will be flammed.
Aside from that remember to post in the right section - perhaps a question such as this would have been better in the trail building section! If you'd looked in there you would have seen that the thread i linked for you is a sticky - bit hard to miss.
Think before you post and you'll get much friendlier responses to your posts