As a thief on the disc, there are a few things you need to know about. Firstly, and most importantly, the rules of economic stimulation (robbing people blind and deaf).
Firstly, to become a thief, go to the guild (a helpful street urchin will take you there if you ask) and, in the foyer, type "join". When you first become a thief, you need to get your hands on a thieves guild licence. After you have one of these little beauties in your hand, the world is your personal bank! The most lucrative place to steal is, of course, Anhk-Morpork, the greatest city in the world!
There are only three big rules which you need to obey as a thief.
If you break any of these rules, rule three will be broken for you. There is honour amoung thieves, so be honourable.
Before going on any stealing sprees, you need to read this or you can easily land yourself in a bit of trouble
With regards to what skills to advance, the most important skills to begin with are stealing and fighting (if you have fighting as a primary). Without some stealing you may find it difficult to satisfy your quota requirements in AM and after all, why become a thief if you're not going to steal? Fighting will allow you to kill for xp, and then allow you to advance your other skills. Backstab is a command which will greatly enhance your ability in a fight and hence working on its skills requirements is a worthwhile investiture of time and xp.
The most important non-primary which you will need to advance is other.health. This determines how many hit points you have.
Onto more mundane matters, the most important NPCs are:
As a thief, you can specialise in different areas of thievery. I recommend doing this to get the most out of the thieves guild.
Advanceing your skills can most easily be done in the thieves guild foyer. Type "syntax advance" to see how to do this. Another good command when in this advancement area is "cost primaries" Also, there are some training rooms downstairs in the guild. Useing these allows you to TM the skills they teach. For more info on TMs, see the help file "help taskmaster".
Lastly, a few things which you should know:
Enjoy the guild, and dont forget to read the other sections of this site to take advantage of all the opportunities offered to you you as a thief.