some questions about team comp in solo queue

So when to pick a certain class (like tank, fighter, assassin...) in any role? whenever i play solo queue i try to pick my champ according to what would be the best kind of champs for the team comp but i find it confusing actually, like, i think 2 tanks is the best but i dunno if they could be replaced by fighters or if 1 tank 2 fighters work or if 3 fighters would make for it. etc. this kind of things.