Periodically there will be a post on this forum looking for players for PBP(Play by Post)Game. When that happens you will need to respond quickly in one of those with your interest and what you would be interested in playing. You typically don't have to have a full character concept ready to go though.
Something along the lines of, Hey I love blah blah and would be very interested in playing a (insert class here) of (very brief concept), I will post a detailed concept later today(insert generic reason for not doing now). If you are early enough(usually this is the key) and have a good enough concept(you can post you are a noob to pbp people are generally very receptive) you will typically get in.
Here is an example of a past post looking for players.