See https://www.wikitree.com/wiki/Help:Membership
At the Guest membership level you can only enter your own information.
At the Family Member level you can enter recently deceased people, and I believe, some living people.
At the Genealogist or Full Membership level you can enter people born after the year 1700. There certifications for pre-1700 and for pre-1500.
Upgrading is easy, and, of course, free. See the link at the top of this answer.