The family go to the 'Gurdwara' (Sikh temple) and get a letter from Gurumukhi (thepunjabi alphabet.) They then use that letter to become the first letter of the babies name, and think of the rest of the name themselves.
If you mean who named Renesmeé, Bella did. Her full name is Renesmeé Carlie Cullen. Her first name is Renee and Esme merged together and her second name is Charlie and Carlisle merged together.