When you first enter the game you go through a newbie tutorial which explains some basic game concepts. You'll choose a class, city and House to join, and after the tutorial you land in the Ring of Portals.
You can read sign to see where the exits take you.
The first thing you should do is introduce yourself to your new citymates. Type something like CT Hello, I am new to this world. You're guaranteed immediate attention, as the community is very friendly and helpful towards newcomers. Introduce yourself to your House using the HNT channel.
After you introduce yourself, a member of your faction will help you train your skills up, they may even offer to mentor you. A mentor is a good thing to have, it's like a sort of buddy system where an older player takes you under their wing.
Don't be shy! Introducing yourself is a good way to get started.
You can start questing / killing stuff in the newbie area. You have a choice of killing imps, pixies or both. The Imp Lord will buy pixie corpses and the Pixie Queen will buy imp corpses, a newbie can make good money by killing both. As you increase in level you will gain lesson points to invest into your skills, you'll also want to save your gold for armor and health elixir. You can buy everything you need in many of the player-owned shops, ask around and maybe you'll get free stuff.
PvE is fairly straightforward but PvP is very complex with hundreds of attacks and cures.
At level 21 you are no longer considered a newbie, but you should be fairly into the swing of things by then. Good luck and have fun!!