assuming you're not a subscriber yet, you can only steal from npc's (non-playing characters)... that is, the computer.  you have to have the stealing skill (which i will also assume you have).  from there... you have to be inside city walls, but not in the city centre or novice districts (magicians', fighters', and thieves' districts are fair game, with thieves' being the 'easiest' to get successful steals).  then, in your drop down menu, you click "try to steal".  sometimes you will find money, sometimes you will get beaten up by mobs and stolen from.  only at about level 25 stealing will the money you make be greater than the money you lose.
if you want to steal from people, you must subscribe.  only then can you click the green flags you'll see beside some people's names and have a chance at stealing from them.
--jason