To start a roleplay, you need to start a board or chat room titled to inform what kind of RP it is. Pirate? Fantasy? Genetic enhanced teenagers? This should attract those who are interested in the subject.
Your first post should lay down ground rules. Followed by either your character introduction or an overview of the setting and details they should know about the world that the characters reside in. Is it a magical forest or in Manhattan? Is there a war going on, a prophesy, an evil doctor hellbent on world domination?
Then you just wait for people to come and ask to join. Either you accept them, or, however, if you have to many people to keep track of, you can deny them. Some people might not even ask, instead, becoming what are called "lurkers" who just watch and might make comments.
When you have enough people, if you haven't posted your intro yet, you can announce that you are starting and thus write and post it. Others will follow after with their character introductions. When everyone's done, most people will try to have their characters interact with one another. Sometimes the person who started the board will create events that will stir up action and keep the characters interacting with one another.
Everything I know about roleplaying, I got off of neopets. I know that this stuff is really confusing, but I think the best way really to understand it is to watch others roleplay and just get the hang of doing it yourself.