Typically with threads like this, the best way to include yourself into the thread is to read a few posts ahead of where you plan to make your entrance then weave your characters entrance into the thread as best you can. It helps to incorporate things that have just previously happened so the other players have something to work off of and then off you go. 

The best way to interact is to give the other players as much to work with as possible. What is your character doing? Are they thinking anything interesting?  How are they interacting with the world around them? Just stuff like that. Before you know it, you'll have a nice post people can work with.