Once you've figured out who you want to play (race, gender, occupation/aspiration, skillset, etc.) you can decide if you want to join a Faction or not. There are two types of Factions: Minor & Major.
Minor faction are groups of players that participate in roleplays with each other and others. It's what you would think of as any other guild, clan, etc.
Major factions are everything a Minor Faction is with the added benefit/responsibility of territory.
If you go to the top right of your screen and click "MAP" you will see the current territories and Major factions. Territory control is a decent measure for activity, sometimes, but you should join a faction that fits your character instead of making a character for that faction. As big as some of the Major Factions are right now, we've lost a few large ones. The map and player base is always changing.
The reason I bring up Minor & Major factions is because that's your easiest gateway into roleplay on the boards. Everyone will put their best foot forward and ask for you to join when you start shopping around for your first faction - don't feel pressured to join any in particular, take your time.