In order to have a custom avatar, you need to have x number of posts. I believe it's 30, but I could be wrong. There's some other surprises along the way you'll encounter as you continue to make more posts. For example, the reputation system is quite neat. If you see another post you like, you can give that person some rep points, and over time you'll gain rep points and thus have more to give. There's all kinds of other surprises, I won't spoiler them for you. Have fun!
As yardgames said, the system is set up to give you rewards as you post more, the more intriguing your posts and the more reputation points you gain, you can bump up the ranks and get more hidden powers, such as custom avatars etc.