Lee Sin is a dominant jungler (as well as laner) that excels in ganking. He is very difficult to be good with, but his 7 abilities can be extremely powerful if you can learn to use them to their full potential.

Smite: This is extremely optional, especially because you will be jungling. Instead of the ones I have chosen above, Revive and Rally are my summoners of choice *insert sarcasm here*.

Flash: This spell sucks. Don't Use it. Not useful for any reason ever. (I'm joking, use Flash)

Armor Pen Reds and Quints: For penetrating armor.
Flat Armor Yellows: For flat armor. Useful in the jungle.
MR/Level Blues: For resisting magic.

I always start with wolves, leading with Tempest, waiting for 2 auto attacks, then using Cripple. Because of Lee's passive, Flurry, it is important to space out your abilities to maximize the bonus attack speed. Next, I go to wraiths, using Smite on the big one, then using Tempest, Cripple, Safeguard, and then Iron Will. Again, make sure to wait for 2 auto attacks in between each ability. After, I go to the golem camp. This time, because you have time to use Safeguard before Tempest, lead with Safeguard/Iron Will, then Tempest/Cripple. You can use Safeguard on yourself. Because Iron Will gives you additional lifesteal and spell vamp, make sure it is active before using Tempest or Smite. After I recall, I purchase a Cloth Armor, then head to blue, always leading with Safeguard/Iron Will. Depending on your runes and masteries, you should be able to clear the entire jungle on your side, excluding dragon. If your runes are not complete, I would recommend skipping red until after you recall again. At level 4, after you get your first point in Sonic Wave/Resonating Strike, you can use that to get to the jungle camps faster by using it to path through walls. After my second recall, I pick up my level 1 boots, 2 pots and a ward. Grab red if you didn't previously, then find your first gank. I prefer ganking top, but if you just got red and are closer to bot, go there.

There are many ways to gank with Lee, but the most powerful combo is W-W-Q-R-Q-E-E, then proceed to spam Q and E when they are off cooldown. You want to activate Safeguard/Iron Will first in the brush, then hit the enemy with a Q and use your ultimate. After, use your Q again to fly over to him, as Resonating Strike deals damage based on the enemy's missing health. Use Tempest/Cripple to slow your enemy and then proceed kicking him in the face as he attempts to retreat. As that is the optimal combo, using Resonating Strike before your ult will allow you to close the gap sooner. Also, try to ult the victim towards your teammates, instead of kicking him towards his tower. Both your Q and W are great escaping skills

Try to land your ult to hit everyone on the enemy team, as well as break up their formation and ult stragglers into your team. Tempest/Cripple is great in team fights, as it hurts and slows movement and attack speed of all enemy champions in it's range. You are a tanky dps (despite your lack of a shirt), so don't be afraid to take a few hits. Also, try to have Iron Will up as much as you can for the extra health, lifesteal, and spell vamp. You can also Safeguard to friendly champions and wards/minions to escape.

Something I personally love doing when I am fleeing from enemy champions is to use my Wriggle's Lantern to escape. I run towards Baron/Dragon, go into the camp, place a ward on the high ground, and use Safeguard on it. This tactic can also be utilized with other champions such as Jax that can jump to friendly wards.

Always try to space out your abilities to maximize his passive. His abilities and style of gameplay are very hard to master, so work at it. Lee is one of the most potent junglers and gankers, so if you can learn to be good with him he can be very powerful.