Hi lelo4851,
Welcome to the forums.
In terms of joining a session, it's a similar method to running a single-player driver session, but instead you select a multiplayer session instead. There are posts in this forum detailing which sessions are available and on which routes. You will probably have to download the route/session involved, and unless you have a First Class Ticket, it will take a few days to do that.
As for hosting a session, it's best to try taking part in one first so you can understand how Multiplayer works.
Shane