Hey welcome to the boards I hope you enjoy your time here. I saw your character is already posted, what you'll want to do next is post in the Sith sign up thread. Once the Sith leadership take a look at the character you'll be able to RP as your Sith and take on missions. I'm a border alliance man myself so I can't give you too much info on the inner workings of the Sith but I hope to see you on the front lines soon. I'm sure other members will pop in soon and give you much more useful information complete with links and such.
Welcome!
Make sure to read this as it probably holds the answers to most of your questions. If any of them are not answered there, please feel free to message me with any help you might need! Otherwise, enjoy your stay.