As a mid assasin tou want to trade in laning phase. Mid game you should have bought atleast your core items (2 items and boots). You shoud look for roams if your lane is even or won. If not - wait for jungle or support for help!
Tips: Lvl 1-Your Q costs 130 energy. You should farm carfully and wait for lvl 3 pokes and potential kills.
Lvl 6- If you are confident enough and think you can do a combo (R1-E1-Q-E2-AA-R2), go for it!

You should play around objectives and always be in the fights. Akali is ver mobile so she can get in,execute a squishy carry (even sometimes a tankier one), like an ADC and get out safely. One thing to worry about are stuns or CC, so be carefull for that.

This is an optional tab! Here you can understand how does your champion actually work.So first Akali is a pretty bad early game champion. You should wait until lvl 4 for good trades. Until then-farm peacefully. You can poke champions with Q-passive. Level 6 is all in level as you can do a combo (shown before in the guide) and execute enemies. As you play more you can see if you can actually kill them or first poke them to lower health as your R2 does more damage to low hp champs. Easy one-shot champs are - veigar, viktor, vex,qiyana and lux.
Lets talk about runes:
Conqueror: Easy to proke, because of your poke and good synergy with the champion.
Precense of Mind: Great for early game as it gives you 1.5 faster energy regen.
Legend:Tenacity: As i said any stun or cc counters Akali so tenacity helps a lot countering that!
Coup de Grace: Great synergy with R2 and E2 with Lich Bane build.
Resolve: Because of you weak early game it is important to have sustain in lane with resolve.Here i can actually show you when you can change some of the runes.
Second wind: This helps vs poke picks like Twisted Fate, Annie and Lux. If your lane doesn't poke you can go for late game pick: Conditioning or more sustainable pick: Bone Plating
Unflinching: This helps with stuns,CC and slows. If you don't need it too much you can for late game: Overgrowth and early game: Revitalize.
Electrocute: Easly proked and good with Akali's kit.
Sudden Impact: Akali is one of the most mobile champions on the game and this goes well with her kit. If you don't think it's good you can go with Taste of Blood.
Eyeball Collection: With every takedown you earn 2 AP which, well gives you more damage!
Ultimate Hunter: This gives you more Ability Haste on your ult and you can execute faster squishy champs. If you play against more tanks though you can go with the Relentless Hunter as you can poke easier.