Hello, I hope you are enjoying the game. It gets very addicting. I will do my best to help you.
Here is a map of Jangan, please view it
http://images.mmosite.com/sro/images/database/npc/jangan.jpg At #28 you can go to the thief town to become a Thief.
In Constantinople [sorry no map], by the Harbor Building you can also teleport to the thief town.
Talk to one of the NPCs there to do it. You will need to create an alias for your job, and pay him 10k or 100k(?) gold, forgot. And then buy a thief suit which cost 10k gold. You must leave all party before you put on your suit. And then after you can only join thief parties.
Thieves cannot attack normal players, they can only attack Merchants and Hunters.
For joining Hunters, you can go to any town, and join the hunter guild at the "Hunter Association," marked on your in game map. Press M to open your map.
For joining merchants guild, you can go to any town, and join them at the "Specialty Shop" marked on your in game map.
***To force attack another player HOLD the ALT KEY and double click on the person you wish to attack. However, there are harsh penalties for this, and doing so is ill-advised. The penalty consist of becoming a "Murderer" which makes your name turn pink/red and anyone is allowed to attack you without consequence. Furthermore, if you happen to die under pink/red name, the probability of losing a worn item, or item in inventory is very high. You will also lose a considerable amount of SP and EXP. You will not be able to stall in any town; and your life drains constantly by 10%(?) while in town. There are two ways to be rid of Murder status. 1) Die by player or by monster; you may need to die multiple times depending on the degree of your murder status. 2) Kill monsters, Monsters in your level range will take off anywhere from 1-7 murder points. When you reach 0 points, you are no longer a murderer.
The above post also contain accurate information. Which is why I left it out in my post. If you have further questions, feel free to ask.