Welcome to the website.
Firstly I would create a sub-account for the character you want to roleplay as. If you're planning to play a pre-existing character then that's perfectly fine. Just name the sub-account with the name of this character and then you can post as that character in threads/stories that are posted on the board. Then I would head over to the biography section of the website and begin manufacturing your character's biography. I would post this biography with the sub-account (rather than your main account).
As you build your character's biography I would have a think of how to fit them into the websites story. If you look at the top of the website (under the 'Articles' tab) you can find a timeline of events. This details all of the major galactic events as well as the present and past day circumstances. If you are looking to find other people to roleplay with you can then find Factions (also at the top of the website) that are full of people (and their characters) to write with.
We tend to keep in touch with each other (and organise roleplay) through a chatroom application called Discord. I would download this, and when you have found a Faction that you are interested in roleplaying with, I would contact a staff member to ask for an invite to their respective server. You can find out who are the staff (in a faction) by browsing their homepage. There is a section displaying who are the staff and who are the other members that joined the faction.
Public roleplaying threads are just what they sound like. If it is posted in that section of the forum then it is open for you to join and participate in. Feel free to read up on what people are writing about and get involved at your own pace. The private roleplaying section are also what they sound like. These are stories usually told between two people (or a small group) that is meant for them only.
There are some other features you can use to create your character. There is a Flair area (found in the settings page for your sub-account) where you can select up to three 'flairs' which let other people know something about your character right off the bat. There are a lot of flairs to pick from so I am sure you can find something to your liking.
I hope this helped.