You can represent yourself in your appeal if you wish, but most people who appeal obtain representation. You can choose to be represented by a Veteran's Service Organization (VSO) or your state's veterans department. Most VSOs have trained personnel who specialize in providing help with claims and appeals. Your local VA office can provide a list of approved veterans appeal representatives in your area.
You can also be represented by a private lawyer or recognized agent.
If you want representation, fill out a VA Form 21-22 to authorize a VSO to represent you, or use VA Form 22a to authorize an attorney or recognized agent to represent you.