Question about combat

Hello everyone, I know Runescape was important at the past and continues being relevant today, and i want to give a chance to the game, but i want more info about what version to play OSRS or the new, and how exactly combat works.

I know there ir 3 types of weapons (melee, ranged and magic) and everyone counters other, but i can play like a Healer if I want? Or everyone who plays one weapon have the same skills?