levelling up cards

i recently started playing obv n tbh i don’t have a hot clue what i’m supposed to be doing with these damn cards. i have 4 that are UR+ n 1 that’s just UR ?? i dunno if there’s a difference but yea n i have 6 that are SSR. should i just pick three and level only those bad boys up or do i have to do them all 😦 if i put my heart and soul into three cards what happens when i summon one i want more. my efforts go to waste ?? 😓 n what’s this business abt having 2 of each sin huh. i am lvl 16 on lesson 5 if that helps. a crumb of advice or guidance pls 💔