At the start of the story, the teenager steals a diamond ring for a girl he knows, but gets caught because his tactic basically consisted of having his friend distract the sales clerk, smashing the case, and grabbing the ring. After serving some time, he finds that the fact that he has a felony on his record means he can't get most work, so he and a friend (I believe the main character is fairly skinny and his friend portly) join up as professional minions for an evil overlord.
I think I read this somewhere in early 2000, in English, but I'm really not certain. The first set of minion armor was either blue or purple, and he got access to some advanced weaponry. At first, things look like it's going to be strictly comedy, about how incompetent the minions are and how the cartoonish the overlords are, but then people start dying, making the kids realize just how serious the situation is that they got into.