Girls, I bought a bed when choosing a mattress they were all very hard…

Girls, I bought a bed when choosing a mattress they were all very hard. I bought the most expensive one in the store. But now some relatives say it's not good to put newborns on a hard mattress. I also wanted to know what to cover the baby with, born in June. I don't even understand what else I need to add, without a clue, please help me ??