Being a criminal is not easy. We want to discourage criminal players, but 
ofcourse it is a part of any world so players can be it if they want to. You will 
become a  criminal if you do the following:

Attack a guard
Attack a player if the player is not in duel or is criminal
Pickpocket someone and not being sucessful
Cast a destructive spell at a guard
Cast a destructive spell at a player if the player is not in duel or is criminal

Once you become a criminal, you will be so for a certain duration depending
on how serious it was. If you log on, and then log back in, the time will begin 
counting from the top again. Guards will attack all criminal at sight so stay 
away from guards. Also, players can kill criminal players without becoming
criminals themselves, so criminal players can be a hot target for Player Killers. 
Also, when you become criminal, your reputation will go more severely down, 
but your underworld reputation will go up. Also, if you commit a crime when the
time is not done, the new time will be added to your current time left before 
being flagged non-criminal.