Items building

So I'm a relatively new player ive played off and on fir about 3 years and one,thing has always puzzled me,with the items system

You see,ive been,researching how to,build champs for awhile and ive been,trying to,find my niche within league first I wanted to be an ad carry the high damage and,insane kills enticed me but I could never build properly in a,match because I don't understand,how to,start off with buying and,how to end the game with buying items. My highest kill game as an adc was 5/7/10 (awful right?) Then I was like oh I know what would be a,good role jungle but again the rotations,and,item,building eluded me So finally I settled on top lane irelia and I've had some luck but,I want to,understand,more Also the reason,I,did,quit for long periods of time is because of the toxicity I would go into a PvP game and everyone would be screaming nonsensical bullshit at me be cause I,didnt understand I would explain that I'm,new and,im trying to,understand and,I,am,trying to help as much as I,can However most responses I,got were go back to bots and if your so bad at the game then why do you play it began to wear on me but I came back a week ago and im trying my best to be the best player I can