Thieving is a fast but not so secure way to make money. Thieving, as its name says, is about stealing from people and chests. It also involves picking the locks on doors, stealing from stalls and disarming traps placed in chests.
The most basic form of stealing is pickpocketing people. The most basic is a common man or woman, you can either be succesful pickpocketing or the person who are you stealing will turn back and stun you for a few seconds.
Another way of thieving is stealing from market stalls. When stealing from stalls you must protect yourself from the person guarding the stall, but you will gain experience easily. There are many stalls in different places, such as seeds stalls in Draynor village and Fur stalls in Ardougne.
Food is pretty much a requirement for this skill as being stunned by an NPC (non-player-character) that you are stealing from results in a set amount of damage being inflicted on your character. Likewise stealing from market stalls can sometimes result in a full blown fight with a specific NPC enemy from guards to Paladins, depending on the stall in question although these fights you run away from.
Once you've gotten into this skill however it is very easy to level up quickly without incurring any costs as many experienced thieves will steal from the cake stalls in Ardgoune before moving onto a higher leveled target. This eliminates the need to prepare food for training.