Head to the village named "Riverwood" and find a house called "Alvor & Sigrid's House". Inside is a man named Hadvar. Rest until it is after 8 a.m. so that Alvor leaves the house. Sigrid and the young girl should remain downstairs and Hadvar will be upstairs. If he is not, wait and he will come upstairs. Save the game before the next part in case something goes wrong. Equip your best sword or weapon that you want improved skill for, and sneak behind him until he cannot see you. Do a heavy strike and he will stumble to the floor. When he gets up will have no knowledge that you hit him. He also never dies, so it can be done infinitely. Repeat the process to level up as much as desired. This will also maximize your Sneak skill, and whatever weapon type you are using.