Hi I'm a noob and I'm currently interested in the assassins... though, I have no idea who to select since im new to this role.
I tried 
a few weeks ago and I can't stand him. He's very boring to me.
And please give me some tips too since I'm really only familar too 
and 
.
Please help!
Thanks.
Is a excellent choice she is easy to look at and really fun to play.
Learning her rotation is really easy. Q E W R
Learning how to play Katarina is really learning when you can fight and when you cannot fight. Learning how to get through her weak early game and how and more importantly WHEN to roam to other lanes for kills. Katarina sucks 1 v 1, there are very few favorable matchups in this game for kat. Good thing this is not why you picked kat, kat excels at killing weak targets and making use of her passive to eliminate entire teams. Katarina is really more of a executioner than a assassin.
[img]http://i.imgur.com/pWPQOj0.jpg?1[/img]
In the above picture the enemy team left champion select without a single form of hard crowd control. Crowd Control is the only thing stopping Katarina from dominating every single game she plays. Learning who has cc and the cooldown of each cc on the enemy team is extremely important. Notice my assists, it is very low for the amount of time I was in combat. This is one of the reasons people hate Katarina, they feel like you are taking all the kills and showboating if you will. People will flame you if you play Katarina and you do not snowball out of control and they will flame you if you do snowball out of control. This is generally true for any assassin you play.
is a good assassin for a beginner also, but her early game is even weaker than katarina's. Akali was my first assassin and I really enjoyed her, but eventually she grew boring to me. I think it is impossible for to ever become old or boring, mainly this is due to the fact you have to react extremely quickly in fights and make correct split second choices that will decide the outcome of the fight. Every single team fight with Katarina, win or lose is a absolute blast.
I must include that could potentially have a rework coming soon, but I am holding out hope that it will be scraped.
I'd say Fizz or Kat.
Yi is probably one of the best assassin champs you could start with because he is easy to use for beginners (although hard to master). i hope you know that the other ones you listed are not really assassins xD.
Assassins fall off, Yi isn't really an "assassin" imo. Basically when you get a lot of kills make it your job to not die, assassins scale badly into late game and if you start trading your life away your team wont be able to follow up on your power spike.
I would say akali might be good. she can deal a lot of damage and she's not too complicated. get gunblade with her first. use Q and then r and it will hurt a lot. make sure you have all three charges.
