I started League with Miss Fortune, shes really good for adc then I bought Caitlyn who is also an adc character and shes pretty easy to play as in my opinion, I pretty new tp league but do a good job woth Caitlyn especially if I have a decent support, if you buy her, build:
Bloodthirster
Boots
Infinity Edge
Phantom Dancer
Ruunans hurricane (not sure if thats how it's spelt)
Last Whisper
Once you learn how to use her trap and evade you'll deal amazing damage
Ive just bought Katarina and Im learning how to play her, she seems really good so far