You need to be a volunteer before you can be able to change a number of things. I think if you enter own profile or your descendents you can manage them, but not parents or ancestors. Assuming you want to be a member of Wikitree you just need to agree to help build the WikiTree and then you can create profiles to your heart's content, except in a couple of cases such as for living people that you're not on the trusted list for or the parents of people born before 1500. There are also some profiles which are especially protected because they're prone to being tampered with.