When i was in your shoes, I played the hell out of YI, just a fun easy to play champ. But Definitely need to follow what the gentleman said above about playing all teh champs you can! Should be trying out every new weeks champs to get a feel. Usually a "BEST" build for each player and game but there are general builds that you can use on each type that will make due.
ADC- Phantom dancer, Infinite Edge, Blood thirster are a staple.. fill in the rest
Support - Gold per 5 items, (lucky pick, philostone, heart of gold, avarice blade) then for the most part just build these items into there better counterparts at midgame(do not build the end items til atleast 20+ you need the gp5 ) Also lots of wards.. vision is your greatest asset in game
Jungler... armor and pots then boots, and finish up wriggle latern early for a free ward and jungle shreding.. then usually some fo the ADC items arn't bad idea
APC (mid) - you'll want rod of ages, deathcap, and zhonyas hourglass/abyssal sceptor/rylais for some defensive things
Finally top, can vary alot but some tanky early items.. (hexdrinker, mallet,) finished with some damaging items. First you'll want items that help directly counter your lane, then push for damage.
Tanks- Some rush warmogs, but I prefer health + mitigation.. So i like randuins (has heart of gold gp5 first.. which will help ) and sunfire cap, force of nature, if you get mogs (atmas is a good choice for armor and dmg)
Hope this helps, honestly If its been a bad day, i revert to the simpler days, veiger, yi, Mundo cuz there easier and work well at lower levels