Do your best to survive in Slain. When you start out, you're so weak that you can barely hold your head up, but survive well enough, and level up fast enough, and before you know it, you'll be a force to be reckoned with.
Then choose how you want to play. Do you want to protect the little guys trying to level, or do you want to give them a taste of what you had to endure when you first started?
Move with WASD.
Attack with the left mouse button.
Block with the right.
1-9 can be used for any skills you unlock.
Level up by killing either NPCs, or players, or simply run around the map avoiding combat and collecting XP orbs on the ground. The choice is yours, but be weary of any high-leveled players out there. In all likelihood they'll be making the choice for you, by killing you relentlessly.
Building yourself up for survival is paramount. Depending on how aggressive other players in the game are, you might want to consider investing in speed. At the early levels, you can't fight, but you can sure run. As long as it's faster than the other players, which, depending on what level they are, might be difficult.
Killing NPCs will earn you more experience, but it'll leave you wounded and easier to be picked off by other players. So take heed as to which NPCs you want to attack, and when.