First of all, don't call me a tard because I made a non-combat thief and I'm sure you will because you all hate love me. If your thief has a combat skill tagged then just go kill stuff, stop reading this.
First of all you'll want to just level by crafting at first, and bulking up your steal skill to AT LEAST 150% before you can rely on stealing for XP, since you can't kill anything to gain EXP.
Step 1: Once said steal skill is at least 150% make your way to an unprotected town with a single cap or equally un-heavy item in your inventory.
Step 1.2: Make sure there's no militia or they'll just waste you when you get caught!
Step 2: Find an unarmed NPC that's close to an exit.
Step 2.2: If you can, try blinding the NPC to make stealing easier!
Step 3: Start planting the item on said NPC and taking it back over and over until you get caught.
Step 4: When the NPC attacks you ineffectively with their fists, run to the exit grid and sit on the world map until your steal cooldown hits zero.
Step 5: Re-enter, the NPC shouldn't be hostile anymore.
Step 6: Repeat starting at step 3.
If you get killed by another player, who cares? You had nothing to lose other than a single cap/item.