Thief Guild
Quest 1: After joining, you'll get your first quest right away. You need to get 100k gold to finish this quest, after which you'll be let into the guild, with your first promotion.
Quest 2: You're told that there is a forge that could be of use to the guild, and you need to go item-hunting for it. You must get a new Magic Water, dropped by Gorgon. Head to the Deep Dark Forest, kill some Gorgons in the Pit, and return. It wasn't the right item after all, so now get a Pegasus Wing from the mountain top and come back for your second promotion.
Quest 3: This quest requires you to get an Iron Baton, which is forged. Kill some Azure Golems and White Slimes to be able to forge it.
Quest 4: After this quest, I'm sure you've had enough about the Maze for some time.. you must find the Green Nymph and find a new pair of Pearl Leggings. She appears randomly at the dead ends of the maze.. quite a difficult task to find her. Search the western part, and start your search when a new hour has just begun (check chat). Try the western dead ends first, then teleport to the entrance with the Inner Maze teleports. Then try the eastern and upper part of the maze. This Maze Map could help you alot.
Quest 5: You must get an Ornate Fascia from Salena (found randomly in the Plains). You can either go search for here or you can use the Catacomb's Teleport (requires 1020 Int) to find her.
Quest 6: ???
Quest 7: Get a suit of Aqua armor.