Hi i'm a plat 3 no life and i'm here to help. so first of all its great that your going to your community for help because and i really cant stress this enough as your #1 lesson in league the best way to improve is to learn about the game,learn from every movement that everyone makes, they all do it for a reason, what should i have done in X situation or what are the best items to have against their team? your community can help you learn faster u can ask people in game (probably shouldn't at your level however) and there are a lot of very helpful YouTube channels that u could watch and learn things from everyday.also, im gonna say alot of big league words so dont get discouraged because leageu is about having fun and its ok to not be an amazing player because if your having fun thats all that matters but u can have fun improving too with dedication if thats what u want. ; ) 
Theres no site where u can really make builds but u _can_research the "best" builds for certain champs since all the builds and thier exploits are already discovered and the reason that when u do look for them and they don't really make much sense is both a combination of u still learning about the game,which leads to u not knowing why certain items are really what u want to have together yet but through playing and reading the items you'll figure it out. and kinda unreliable build sites. 9 times out of 10 though if u really want to know what you're getting into item-wise the recommended items are usually what u want, if u REALLY wanna know why its good or u just want to get better at the game by understanding more u could type into YouTube best graves build or whatever champs ur into.
THROW OUT THE idea that champs are selectively ad or ap. its not too confusing u got this stay with me. all champs are founded on their abilities. you know, qwer and their passive. well, some champions abilities do magic gamage, and some do physical damage, some do both these abilities scale (or get stronger) based on how much ad and ap you have. all the champions abilities (scale) differently. 
lets say at level 1 fizzes q scales off 50% ap whila jannas only scales** 15%** if they both have 100 ap fizzes q will do 50 magic damage while jannas would only do 15.
of course these are very low numbers its hypothetical and unrealistic a bit plus ontop of using this smaller numbers to hopefully make it a bit easier to understand. if a champion particularily confuses you, it can be very helpful to watch that champions champion spotlight or even play them once ot twice just to understand what thier abilities do.
ALSO if something doesnt make sense read the tooltip in the game if not google it stay safe 
