How do you get free membership with runescape?

Answer 1

You don't. "Membership" is, almost by definition, a number of benefits you get against payment. That's how Jagex (the owners of RuneScape) make their money, and are able to maintain game development, hosting, etc.

Sometimes you may get a membership with some special event - for instance, as a prize. But only very few people get memberships that way, so you better don't count on it. It is much easier to earn the money, to get a paid membership.