Fighting. Pick a monster ( or a click) and hold . Its frame at the top of the screen shows its level and HP. Monsters above your level hit much harder; the red dots on the minimap are monsters.
Loot. Click gold and items to pick them up (from too far, they're taken once you walk up to them). What a monster drops is yours for 15 seconds if you did the most damage to it (in a party, the party's damage counts together).
Your bag. Double-click gear to wear it, drag items between slots, right-click for everything else. Hold Shift over a piece of gear to compare it with what you wear.
Skills. You start with one skill point and get two more every level: click the gold + on a skill in your skill bar, or use the skill window. Later, Skill Books and soul items from Rift Stones raise skills further.
Potions. and drink the potion on that key (a red (HP) one and a blue (MP) one to start with; drag any potion from your bag onto a key to change it); it pours in over 4 seconds, and that colour then waits 15 seconds. The herbalist in town sells them.
Dying costs a little EXP and gold, less with a good reputation. You earn reputation by hunting monsters (more for stronger ones) and breaking Rift Stones, and a little every minute you play at Neutral or better.
Rift Stones hide in every hunting area. They call out waves of monsters as they crack, and drop gear and Skill Books: bring friends.
Parties. Right-click a player of your kingdom to invite them. Fighting near each other gives more EXP.
Travel. The teleporter outside your town takes you to the Highlands from level 5; the desert opens at level 10. The stable sells mounts: put one in your mount slot and ride it.
PvP. Other kingdoms can always be attacked outside safe zones; your own kingdom only with PK mode on.