Ban Fizz, since he's probably the most unfair of them.
Assassins tend to go through bursts of popularity and then fall off, so I really recomend you pick up midlaners that are strong vs assassins to play during those periods. ESPECIALLY while you're still learning.
Examples:
To a lesser extent, Orianna but she's not really a counter so much as she is a skill matchup.
As for ingame:
The first thing you need to understand about playing against Assassins is that it's this really boring not very fun game of "Make sure they don't get fed." Most Assassins are meant to dominate the early game, and then fall off hard. So you can set yourself back as much as you want, as long as they don't get enough kills, a Zed building the right items is weaker then an Orianna with
.
That being said: Not all assassins are like that. Fizz, Kassadin, and Yasuo are meant to be strong in the mid-late game, and a lot of this won't work too well against them. To a lesser extent: Ahri's meant to be more of a mid game threat, but I still think most of this will work against her.
Take teleport, leave lane often and don't let yourself get low enough for them to assassinate. Consider taking resolve as your second rune tree.
Against AD assassins, build Doran's ring > Cloth armor > then another doran's ring.
After that, your next item depends on how well things are going, how comfortable you feel, and what kind of jungler you're against. If the enemy team has a second assassin in the jungle, Zhonya's might not be enough. Especially if the mid lane assassin has a kill. Mages with good CC and teamfighting potential also have the option to build warden's mail and then finish it latter into a frozen heart. Damage Centric mages should probably NEVER do that though. Except maybe Karthus. Karthus is weird.
to wrap this up:
Either A)
into
into whatever your normal build would be.
or B)
as your mana item into a health/AP item
/
/
Note that this REQUIRES Manaflow band. But you should probably have that vs assassins in most matchups anyway.
Against Leblanc, doran's ring >
> Doran's Ring >
.
Leblanc is, again, 100% built around dominating you in lane. But as long as you stay out of her distortion range, she has to rely on either Jungle ganks or poking you down by distorting into range to cast her other abilities until you're low enough to kill. The MR and health regen from Specter's Cowl does a lot to mitigate that. Even more if you take Resolve as your second rune tree.
After that,
, normal build or focus on Health/AP items depending on how you feel.
This is a lot harder vs Ahri, but it does still slow her kill potential tremendously. Just watch out for her after level 6.
Aside from that, try to keep the bushes around river warded and ping "enemy missing" TWICE when the assassin leaves lane: Once mid, and a second time do it near your ally who the assassin is heading towards.
Yasuo is way more matchup specific. my best advice is to just pick anti assassin characters, play scared and far away from him, and just try to learn. Also, build
early to help dodge tornados.
Just ban Fizz.
Kassadin doesn't need much specific info. Just bully him before he hits level 6 and respect him when he hits 6.
Hope this helps o/