Welcome to my first Guide. I am fairly new to LoL so please bear with me.

Lee Sin

As a young teen, Lee Sin was intent on becoming a summoner. His will and dedication were unmatched by any of his peers, and his skill drew the attention of Reginald Ashram, the League's High Councilor at the time. While studying at the Arcanum Majoris, Lee Sin became frustrated with instruction paced for the other students. He spent his free time researching the nuances of summoning in hopes of graduating sooner. He made amazing advances in his arcane studies, surpassing all other students. By all indications, he would have become one of the League's greatest summoners were it not for one terrible mistake. Too impatient, he attempted to test his ability by summoning a beast from the Plague Jungles. What he summoned instead was a young boy, but not in one piece. He barely had time to look the boy in what was once his face before the jumbled human mass fell lifeless to the floor. A League investigation later revealed that the boy's entire village was obliterated by feedback from the ritual.

Lee Sin's talents were so promising that the League was willing to overlook the incident, but he could never forgive himself. He left the Institute and journeyed to the Shojin Monastery for eternal repentance, swearing never to practice magic again. Years later, hoping to atone for his crime with martyrdom, he set himself ablaze as a protest of the Noxian occupation of Ionia. He remained alive in this state, enduring searing agony for weeks. His actions paved the way for a League match wherein Ionia prevailed, but by the time he was doused, his eyes had been burned completely from their sockets. Hailed as a savior, he was reborn, and his will to act invigorated. He joined the League of Legends to continue his atonement with sweat and blood, a true monk's only possessions.

"The actions of one may sunder the world, but the efforts of many may rebuild it." - Lee Sin

Stats:

Damage 55.8 (+3.2 / per level)
Health 428 (+85 / per level)
Mana 200 (+0 / per level)
Move Speed 325
Armor 16 (+3.7 / per level)
Spell Block 30 (+1.25 / per level)
Health Regen 1.25 (+0.14 / per level)
Mana Regen 10 (+0 / per level)

For Items I start with the Vampiric Scepter for the life steal. I ALWAYS follow it up with the Mercenary treads for the magic resist and reduction on Slows, Stuns and what not. I then Finish my Wrigglers Lantern and start to jungle if my team doesn't have a jungler. I usually refer to this as simply "Late Jungling" since for me personally it's hard to jungle at the start of the match when i'm only summoner level 5. After Wrigglers and go more more movement and attack speed by getting a phantom dancer. It boots Lee Sins move to about 446 so no one can really our run him easily once he has that. After Phantom Dancer I go for the Guardian Angel for the armor, magic resist, and it's passive. He makes Lee Sin able to take a lot more damage in fights, and has saved me many times. Once I have obtained that I go for the Ghost blade because it's armor pen, and Unique active of booting Lee Sin's attack speed to above 2.0 and movement speed to about 530ish ensures that Lee Sin will get the job done! Finally I go for the Frozen Mallet. The Health bonus is nice but the slow is a lot better, with his attack speed at about 1.5 and once a minute able to go above 2, and his movement speed of 446 and once a minute able to hit 535, when ever he hit's someone with a physical attack, why not slow them as well so the rest of your team can catch up.

Description of Skills:

Sonic Wave / Resonating Strike - Sonic Wave: Lee Sin projects a discordant wave of sound to locate his enemies, dealing physical damage to the first enemy it encounters. If Sonic Wave hits, Lee Sin can cast Resonating Strike for the next 3 seconds.
Resonating Strike: Lee Sin dashes to the enemy hit by Sonic Wave, dealing physical damage plus 10% of their missing health.
Sonic Wave: Lee Sin projects a discordant wave of sound to locate his enemies, dealing 60/90/120/150/180 (+) physical damage to the first enemy it encounters. If Sonic Wave hits, Lee Sin can cast Resonating Strike for the next 3 seconds.

Resonating Strike: Lee Sin dashes to the enemy hit by Sonic Wave, dealing 60/90/120/150/180 (+) physical damage plus 10% of their missing Health (Max: 400 damage vs. Monsters).
Cost
60/60/60/60/60 Energy / 40/40/40/40/40 Energy
Range
975

Safeguard / Iron WIll - Safeguard: Lee Sin rushes towards a target ally, shielding them both from damage. If a shield is broken, half the Energy cost is returned. After using Safeguard, Lee Sin can cast Iron Will for the next 3 seconds.
Iron Will: Lee Sin's intense training allows him to thrive in battle. For 5 seconds, Lee Sin gains Lifesteal, Spell Vamp, and Armor.

Safeguard: Lee Sin rushes towards a target ally, shielding them both for 40/80/120/160/200 (+0.8) damage over the next 5 seconds. If a shield is broken, half the Energy cost is returned. After using Safeguard, Lee Sin can cast Iron Will for the next 3 seconds.

Iron Will: Lee Sin's intense training allows him to thrive in battle. For 5 seconds, Lee Sin gains 10/15/20/25/30% Lifesteal, Spell Vamp, and 10/15/20/25/30 Armor.
Cost
60/60/60/60/60 Energy / 40/40/40/40/40 Energy
Range
700

Tempest / Cripple - Tempest: Lee Sin smashes the ground sending out a shockwave that deals magic damage and reveals enemy units hit. If Tempest hits an enemy, Lee Sin can cast cripple for the next 3 seconds.
Cripple: Lee Sin cripples nearby enemies revealed by Tempest, reducing their Movement and Attack Speed for 4 seconds. Movement and Attack Speed recover gradually over the duration.

Tempest: Lee Sin smashes the ground, sending out a shockwave that deals 60/95/130/165/200 (+) magic damage and reveals enemy units hit for 4 seconds. If Tempest hits an enemy, Lee Sin can cast Cripple for the next 3 seconds.

Cripple: Lee Sin cripples nearby enemies revealed by Tempest for 4 seconds, reducing their Movement and Attack Speed by 30/37.5/45/52.5/60%. Movement and Attack Speed recover gradually over the duration.
Cost
60/60/60/60/60 Energy / 40/40/40/40/40 Energy
Range
350

Dragon's Rage - Lee Sin performs a powerful roundhouse kick launching his target back, dealing physical damage to the target and any enemies they collide with. Enemies the target collides with are knocked into the air for a short duration. This technique was taught to him by Jesse Perring, although Lee Sin does not kick players off the map.
Lee Sin performs a powerful roundhouse kick launching an enemy champion back, dealing 200/400/600 (+) physical damage to the target and any enemies they collide with. Enemies the target collides with are knocked into the air for a short duration.
Cost
No Cost
Range
375

Flurry - After Lee Sin uses an ability, his next 2 basic attacks gain 50% Attack Speed and return 10 Energy each.

The order I use from lvs 1-3

I usually start of with Safeguard/Iron Will so I am able to defend my ally when laneing and get better life steal to keep my hp up. I follow that up with Tempest/Cripple so I am able to stop my enemies from escaping quickly and am able to hopefully get early kills. I then get Sonic Wave / Resonating Strike to close distances and get more kills.

After that depending on the situation depends on what I get, if i need more survivability or to help my team more I go Safeguard / Iron will, Need to slow enemies more I go Tempest / Cripple, and need to close gaps more often, and deal more damage. Sonic Wave / Resonating Strike.

For Summoner Spells I go Exhaust and Ghost, to slow enemies and speed my self up.

Exhaust is grate for slowing the enemy so it's harder for them to escape and on top of Cripple makes them barely able to move for a few seconds which can usually ensure a kill.

Ghost is awesome for Lee Sin because from what I know he is one of the fastest champions and ghost as well as being able to dash around from enemy to enemy or ally to ally makes sure almost no one ever escapes a fight as well as being able to escape himself if need be rather quickly.

In early team fights I usually use Lee Sin for his In & Out burst damage. Sonic Wave / Resonating Strike to get in the middle of the fight. Tempest / Cripple to hit almost everyone then Safeguard / Iron Will to get out, help an ally and regain health faster.

One I get Guardian Angel I usually allow Lee Sin to take more hit's since he is now somewhat of a sponge (able to take a decent amount of damage). I will use Sonic Wave / Resonating Strike to initiate or to throw myself into the fray and also catch enemies trying to escape or back off. Use Tempest / Cripple for AOE and to slow enemies so they can't escape. and then Safeguard / Iron Will to shield allies, Back out if need be, or regain health with life steal, as well as keeping myself in the fight longer because of the shield.

I personally don't use Lee Sin to jungle (i am only Level 5 at the moment) but if my team doesn't have a Jungler I will usually start jungling around level 4-8ishs. Usally right after I get Wigglers.