Hai. I'm a LoL gamer. I happen to have more than 100 games with Akali. So I thought it would be good to share this guide I made up myself, to the rest of the community. If you are to troll, thou shall be ignored, reported and flagged. If you don't like it, tell me why, don't troll.

FOR NOOBS:
CDR - Cooldown Reduction
AD - Attack Damage
AP - Ability Power
BD - Burst Damage
DPS - Damage Per Second
CC - Crowd Control
MPen - Magic/Spell Penetration
MR - Magic Resistance
ArP - Armour Penetration
AS - Attack Speed
HP - Hit Points aka. LIFE (If you didn't know this, you should prolly quit LoL.)
MP - Mana
E - Energy

I'd mainly vote for the classic 21 - 0 - 9 AP Carry/Assassin build. Especially with those new masteries, Akali is literally OP with the +5% AP and the extra damage when target is below 40% HP.
I take Utility masteries cause now, with Flash's spell, it's really important that it comes upgraded with its mastery. For more info. see the Spells' section.

-Marks of Insight/Quints of Insight.

Mag.Pen.!!!! What else could anybody recommend as reds and quints? They're the most useful runes, for assassins (but Talon/Shaco and bruisers)/ap carries.

-Seals of Potency.

Here someone could be elastic about their choice. Seals can include Armour, Energy Regen (not so recommended for Akali, since she isn't as costy as Kennen or Shen are when it comes to Energy), Dodge (Out of the question, it's no use buying them now, they're getting removed) or Mana Regen (Not at Akali, ffs).

-Glyphs of Focus

A must on Akali. This extra CDR will provide her with enough spam as to be able to harrass the enemy carry/assassin/support/cc and tear them apart. The only downside on them is that they are really costy. If one have them, they should be greatful, if they are to follow this build.

WHY DO YOU MAX OUT Q FIRST?

Cause it's damage is OP. Dealing about 400 - 500 damage, add the auto attack that will refund you more and more in terms of spell vamp and energy.

HOW DO YOU OWN SO MUCH WITH AKALI? HOW DO U ENTER THE TEAMFIGHT?

It depends, on the situation. Mainly, you either have to be lucky as **** or be really observant of the teamfight. You have to enter at the correct time, wait for the ccer to stun, or wait for the Singed to Fling, in order to go in (for example).
You have to focus the AP Carry or the Support, depending on each others' cooldowns on skills and tear them apart.

Example:

A Soraka just Wished for the Olaf in jungle. She healed Annie and silenced our Brand. Meaning she has cooldown on 3 skills so far. So you are free to enter the fight, W inside them and harrass the AP Carry.

Example #2:

The enemy Katarina used Death Lotus. Enemy Alistar has Headbutt and Pulverize but Sona still has her ult up. Your team is falling back. DON'T FALL BACK. W inside them and wait for sona to come close. Q -> Auto attack her and wait. They will ult you and try to stun you. But, you're stealthed, so Sona has just wasted her ult and you are free to **** her up. ^^ Ownage, enemy support dead, teamfight win.

FLASH

A must on Akali. Can help her escape, reach an enemy, Hextech him and get him to get focused by her team.

IGNITE

An okay spell, especially after the new patch that deals 450 true damage to the target. If they don't heal or so, guarantees a kill without any deaths by mistakes. Useful.

TELEPORT

Another okay spell, if you're solo top vs 2. Can help and even turn a bad situation in someone's triple kill or can help you def or so. They only reason I don't take Teleport is that I can't use it in the teamfight so, it basically rids me off some Utility and damage, which are my goal to develop through the game, AS AN ASSASSIN.

HEAL

When vs meelee attackers like Yi or Udyr, jungling. Don't recommend it, you'll get flamed if taking it, especially on Ranked. Add that it's useless at late-game.

LANING

I mostly pick solotop with Akali since 1.She NEVER misses a creep. 2.She's an easy farmer with burst damage. 3.She can be a beast vs most champions.
Akali can still go at mid although, as an assassin, her burst damage is whatever it takes for her to own the hell out of the enemy AP carry.

INSIDE TEAMFIGHT

ALWAYS engage the enemy support or the AP carry. NEVER turn at the AD carry, let your team's AP carry and AD carry take care of them. If the support or the enemy AP carry fall, your job is technically done cause the enemy team is lacking either burst damage or healing meaning that they are missing ONE THIRD OF THEIR UTILITY.

HOW TO LASTHIT - ENGAGE

LAST HITTING is EASY with Akali. Simply, Q a creep when mid HP and when it's below 30% HP auto attack it and kill it. Qing and hitting back is EASY and VIABLE with Akali since she DOESN'T HAVE MP but has ENERGY. Add the 40 energy regen, when rank 5 (meaning you're lv.9), it's pretty much free farming.

ENGAGING is kinda tricky. As mentioned above, you have to keep in mind enemies' crowd controlling skills like Xin's Q or Singed's E or Orianna's Ult. But you'll get to learn it through, eventually. Practice makes perfect.

There's what you've been waiting for and the reason I put it in the bottom of the build, so that you read the rest and have a full view of Akali.

-Start off with Boots of Speed and 3 HP Pots.
-When back, make your Hextech Revolver and buy some more HP Pots.
-After that, finish your Mercury Treads
-Finish your Hextech Gunblade for its cool active and go beat the **** out of the support or the AP carry.

By now, it should be around 20:00 add or take 5:00. Otherwise, ur doing it wrong and u need to retreat to lane farming and jungling, and avoid teamfights.

-Buy a Giant's Belt.
-Buy a Needlessly Large Rod and a Blasting Wand, if you can afford it.
-Finish your Rabadon's Deathcap and get a Sheen.
-Finish your Lich Bane.

By now, you should have 25+ kills, 10-hit everything and be able to pwn like hell.

-Finish your Rylai's.
-Get Elixirs and Oracles'.

Last item should be anything that provides AP. It depends on your enemy team.

>If they have a heavy AD DPS team, get Zonya's.
>If they have a heavy AD Burst team, get Abyssal Scepter.
>If they have a tanky team, fet a second Rabadon's Deathcap.
>If they have a heavy MR team, a Void Staff wouldn't be half bad.

REMEMBER: BUY ELIXIRS. They tend to get so underestimated but grant 10 AD 40 AP 7% AS 240 HP 10% CDR 9% CRT.STR. and the ability to see stealthed units. Pretty much win situation, especially the AP + 35% of 40 = 54 AP.