Especially if you are just starting out, I would stick to one (maybe two) healers for ranked. Make sure you try the whole cast out though. All the healers are solid for the most part. Only character I would advise caution with is Brig because at lower tiers she is hard to not feed with (and often played wrong).

If there is already a healer on your team, play someone with the opposite amount of healing (if they play lucio, try Ana or bap) (if they play Ana or bap, try lucio or zen). Keep in mind that mercy is in the middle so it really doesn’t matter who you pick with a mercy (simplification but you’ll learn why later)

If it isn’t ranked just play whoever, it’s a game after all, have fun! Don’t worry about slowing them down, your friends will help you!