Make sure you have been paying your loan on time. The only way you are going to be able to get out of having a co-signer is if you have been paying that particular student loan on time for an extended period of time.
There are two ways to remove a co-signer: you can refinance the loan or contact your lender.
Refinancing is the process of getting a new loan at a lower interest rate to pay off the original loan. Loans can be refinanced with the original lender, or you can get a loan from another lending institution which may offer better rates and terms.
Another way to get rid of a co-signer is to contact your lending institution and request the co-signer be taken off. The lender will run your credit, and if it is sufficient, you will be able to take on your loan individually.