You know, usually people don't *choose* to become a thief, they do it out of necessity. It's a hard life. You always need to hide in the dark, lurking for some potential victim to appear, preferably a good victim: weak and rich. You think that's immoral? Then don't be a thief. That sort of morality is the privilege of the strong, the fighters and the high mages. If you're weak, you can't afford morality. Be clever instead.
Of course, if you're successful, you'll become rich. Very rich.
You'll be able to purchase the best items in town, use the underground network to get information on potential victims and to reach those who think they're in security because they paid for protection (you can buy anyone's loyalty - so much for the fighter's "morality").
How do you manage all this? There are essentially two things you need to pay attention to: your money, and your victims. You need to be a good manager, sometime waiting to get some more gold for a better item of equipment instead of spending all you have immediatly. And you need to follow your victims closely, know exactly how strong they are - the risk - and what they're worth - the gain. You need to really stalk them. Know exactly where and when they will be found and be there before anyone else is. But don't become visible too fast, or you will draw attention and someone else might come to try and take your gain.
Stalking becomes more and more important at higher levels. This is especially true when your worthy (in terms of gain) foes are for the most part very strong, but that you can't afford the best weapons yet.
When must you properly "steal"? At low levels, this makes sense against your personal enemies, to acquire some experience - learning the tricks of the trade, so to speak, but later on, this becomes utterly useless. Stealing, after a while, is worth it only in the forest, the goal beeing gold - but choose wisely what foe you're going to steal from. "Know thy enemies", and choose to steal only from those that will give a good gain in gold. Be warned, though, that half the time you'll not get any gold at all, which counts as just a petty experience once you've become a master thief. This is very annoying, but it happens to even the very best thieves, twice a day usually (and twice a day you do find good treasure). Notice that you won't be able to steal more often because you're more experienced, you'll just be able to concentrate more steals on a single victim, waiting longer for the best forest enemy to be found.
Well, sooner or later, you'll be a master thief. One of the richest people of the land, you'll have an excellent weapon, and might do some business with those that used to be your foes. You'll wield one of the best blades available to eliminate the most dangerous monsters in the forest, including maybe the fabled Manticore.
Yet know that you will never
be a match to the best fighters of the land. If you wanted to, then you
ought to have chosen another career: fighter or mage. You will however
have an easy life in your old days, and might want to retire, keeping nonetheless
the ability to hoard large amounts of gold when confronted to the forest
during your new life.
Atan, first thief hero of the land.