Mandalorians, as a species, are extinct in Star Wars. Those claiming the name now are simply other species continuing the traditions. We actually have a group here doing that. Corellians are considered different from average humans because those from the planet are inclined towards certain skills. It's the same with the other human "subspecies". I'm not sure who decided to add them, but I think it was to add more variety since so many people just chose human, and all had the same skill bonus.
The market on your menu is to access the NPC market - basically the possessions of dead people being sold by the server. It can be accessed from certain specific facilities, of which there is a decent list on the RPG Centre, in the Traders Lounge forum. You might try a Search for it - located at the bottom of each forum, I believe.
You play from this website interface - most usually the Position and Cockpit screens (once you get access to a ship).
"Clans" are known around here as "factions". You can go to Manage -> Faction on your menu, and join one from there. There's also a list of all of them with some information (and links to their websites) that can be reached from the Rules section, and also from some other pages like the Sim News. Once you join a faction, they can arrange to have you picked up and most will assign you the use of a ship, and anything else you might need while working for them. They'll also give you some stuff to do, though it's advantageous if you can find ways to amuse yourself.
Your avatar can be changed from Manage -> Character, and then editing your profile. You'll need the image uploaded on the web somewhere, not just on your own computer.
____________"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."
Only fools and children dream of heroes.