Hello, Akali is a really OP hero is you use it. I dont like creating such a long guide thus, Ill create a short guide so that you will get a hang of akali. Remember, Practice makes perfect (:

Items. Akali Should be one of the strongest AP mid in mid lane. Her weakness is against tankier opponents who she can't really burst down like gragas ETC.

Start with 1 boots and 3 pots for better sustainability. The key to akali is keeping all your pots, ignite, flash and health up till you reach level 6. SO PLAY SAFE. It's hard to kill anyone before level 6. Once you hit level 6/7, aim to have about 1.2k (to get your hextech revolver). Against a paper ap mid like maybe LB, you should aim to have a stored ulti of at least 2 so that u are able to dive her when she is dying.

Go with R,Q,E,(normal hit to burst), W/ ignite if need. However usually after spamming RQE she will probably flash away or something, proceed with your second ulti and repeat RQE.

Easy to say, but you have to read. If you are owning your lane or getting kills from roaming, get a MEJAI. The feed will make you too hard to catch with mejais.

RYLAIS AND RABADON - I believe rylais slow is very essential. Only when you get rabadons, can you truly burst down most ADCS. (unless you have majais full stack)

I believe most mistake people make is getting to engrossed in KILLING. Yes, killing is good, but roaming makes you spend a lot of time. And when you cant get kills, you are underfarmed.
Always get 3 pots with you for sustain so that you are able to kill and go back to your lane with full health. You should always get the neutral creeps when you are doing heavy roaming, this is to keep your gold up.