Master Yi's strengths:
Has completely ridiculously high damage output
High mobility
Can snowball out of control easily
One of the most efficient split pushers
Farms well
Pretty much the best champion to start playing the game with (easy to learn, extremely powerful at low levels/elo)

Master Yi's Weaknesses:
Brings no utility at all for teamfights (No CC)
Gets countered hard by crowd control
Sometimes can have Mana issues (if you gave blue)
Needs to take a lot of farm to be effective (Less farm left for your allies)
Will be the number one priority target for the enemy team most of the time

Flat AD more burst on ganks and getting that extra damage for your Wuju Style and autoattacks
Armor Seals are a must-have on junglers. They will make a huge difference.
Magic Resist per level Glyphs are a great pick because this build doesn't include much magic resistance, just masteries!

Double Strike

Master Yi strikes twice every 4th consecutive attack. The second attack deals 50% of your attack damage, applies all on hit effects, can critically strike and counts as a hit for the counter of the next double strike.

A free steroid for Yi, what's not to love? You actually don't really have to pay much attention to this passive even though it's awesome. Just know that even though the animation clearly slows two hits, you can still treat it as one normal attack (you don't have to wait before the animation of the second hit starts before you start moving again)


Alpha Strike

Master Yi becomes untargetable and jumps to up to 4 nearby enemies dealing 25 / 60 / 95 / 130 / 165 (+100% of attack damage) physical damage to each unit hit as well as 75 / 100 / 125 / 150 / 175 bonus damage to minions and monsters.
This skill can critically strike dealing (+60% of attack damage) bonus physical damage. Basic attacks lower alpha strike's cooldown by 1 second.

This ability makes Master Yi a good jungler.

You need to max this first as it is your main ability to do... everything pretty much. Don't even think about maxing Wuju Style or even Meditate over this. This ability just gives you too much damage.

When ganking, try not to use Alpha Strike right away, make use of your element of surprise, run up to your enemy to autoattack him. If possible, try to use Alpha Strike only after your enemy has used Flash. You will catch him again and hopefully kill him!

As Phreak mentions with his champion spotlight, with very careful timing it is possible to dodge spells with Alpha Strike since you become untargetable for a split of a second. This is something you should really try to make use of when you're in a close 1v1 fight.


Meditate

Master Yi channels, restoring 30 / 50 / 70 / 90 / 110(+30% of ability power) health per second. This amount is increased by 1% per 1% of health that Master Yi is missing. While channeling, Master Yi gains 40 / 45 / 50 / 55 / 60% damage reduction. This amount is halved against turrets

Yes I sometimes (not always) take this ability at level 4 because it's a fairly good sustain ability and it can allow you to make plays early by tanking an enemy's damage. It can also sometimes save your life. You only lose 1 point in Wuju Style which wouldn't have gave you much anyway.

Late game, you should most of the time avoid to use this because attacking things will usually give you more life back than the actual heal. However, it can still be a great counter to channeled spells if you use Meditate right as the damage comes out. For example you can use Meditate right before a Nunu's fully channels his Absolute Zero or if a Karthus is using his Requiem.

A rather underwhelming spell on AD Yi but not on AP Yi.



Wuju Style

Passive: Increases Master Yi's bonus attack damage by 7 / 9 / 11 / 13 / 15%.

Active: Master Yi can activate this ability deal 10 / 15 / 20 / 25 / 30(+10/12.5/15/17.5/20% of attack damage) true damage per hit. Master Yi loses the passive effect of this skill while it's on cooldown.

This spell makes Master Yi such a monster late game when you invest a lot in Attack Damage. Tons of free damage for Yi yay. During the entire game, you need to be careful with this spell because activating it too early can REDUCE your own damage output. Make sure that when you activate this spell, whatever you were trying to kill will 100% die within the next 5 seconds. Having a great timing can be difficult sometimes and will come with experience. If you are insecure about it, know that activating this spell late is better than too early.



Highlander

Passive: Champion kills reduce the cooldowns of Master Yi's other abilities by 18 seconds, and assists by half this amount.

Active: For 10 seconds, Master Yi gains 25 / 35 / 45% movement speed and 30 / 55 / 80% attack speed and becomes immune to movement speed slows. While active, kills and assists extend the duration by 4 seconds.

Very straightforward spell. The passive is what makes Yi such a clean-up champion and the main reason why it is a good idea to build Master Yi as a full out damage champion.

The active part isn't very complicated... use it when you're trying to kill someone :P. You should not use this to kill jungle monsters or to kill towers (you usually want to save it to get away when the enemy team tries to stop you). Also, it makes you immune to slows so do not be worried about that Ashe or Nasus or Nunu making you useless with slows, it won't happen :P.

With this ultimate activated, your Wuju Style and your Double Strike, simply by right clicking an enemy, you will do unmatched autoattack damage and no one should be able to escape you. Especially if you activated Youmuu's Ghostblade.

Master Yi is not an initiator, or an heavy utility champion with Crowd Control. All you can do is deal damage.

With this build, you are an assassin. You die fast but you can kill enemies fast as well. Your dream is to jump on a squishy, kill him really fast and then keep jumping on enemies one after the other with resets and get a Pentakill. In order to do that, you must not die because... well you can't attack when you're dead. That means you have to be veeeeeeeeery careful about your positioning and the time that you go in.
All you can do is use Alpha Strike, activate Wuju Style,Highlander,Youmuu's Ghosblade and right click a target target. This combo will unleash heavy burst damage but you have to remember you can also die very quickly. If possible, try to target a target that is low on Health already or on a target that is really squishy (will die quickly). That way you will get your job done and makes your cooldowns refresh as well.

If no one on their team gets low HP for you to clean up AND you see that the enemy squishies are way too well protected because their support/tanks are doing their job of peeling extremely well, just try to kill another enemy. Basically, if you jump on the enemy carry just to get stunned and killed in a second, it's not good. So yeah, if the enemies carries are too well protected, most of the time the best thing to do is to kill the enemies that are trying to kill YOUR carries, while not being in the enemy carry's range. Yeah it's not easy to do at all but you can't do anything else or you will die.

In the best scenario possible, no one would focus you. This happens a lot in low elo/low levels.