Anyone can create a page in the user namespace. If you want to create a user sub-page, see the help page specific to that.
Only admins can create pages outside the user namespace. Here are instructions for doing that.
1. Create a link (from any page) to the non-existent page you want to create. (This is usually quite natural since it is odd to want to create a page except when creating a link to it.)
2. Click on the link.
3. Add content to the page.
4. Click "Save Page."
Note if you want to create a subpage take the following steps for step 1. 1a. Create the link in the following format.
Note: replace "parentpagename" with the name of the parent page you want to create a subpage off of. replace "subpagename" with the name of the new subpage you want to create.