How to get the most out of Pull ups/chin ups for beginners?

I just bought a bar for my doorway and I want to use it, cept i have no way to get the most out of it as i can only do about 5-6 pull ups until i get tired. is this even enough? if so, how can i get on a routine to improve myself? if i can only do 5-6, how can i improve to go on to doing 10-20 or more over time? and how many times a week should i do this?