If you just need some general pointers, here they are :).
Admission Essays should be a very straight forward, practical document. First and foremost, do your homework. Know what the university/college expects from a potential graduate student. In addition, it is extremely important that you don't simply try and persuade them how great you are, but actually show it with definitive proof. A graduate committee needs to see everything - not just your grades but your community involvement and personal interests. Another point to make is that your essay should be focused. Do not ramble on about how wonderful you are, but give them a focused, directed statement about what you plan to do with your graduate education. If you can identify a particular professor you want to work with and a plan of research, you have a much better chance.