OK, so what's wrong with your grandmother having custody?
Bottom line is that OP will have to convince the court that he can provide a better environment for the child than the current guardians/custodians.
Even with a home that's paid for, I have a hard time believing that an 18 year old living on a Walmart income is going to be able to provide the level of support needed. It's not just a money issue- I doubt very much that the maturity is there to bring up a pre-teen kid.
But go ahead - tell us why the child is better with you and we can tell you if there's any chance that it would pass muster with a judge.