Good morning,
It depends on several things, but there are more than one way to do it.
If there is (and will be) only one page, the page does not get reloaded often, and if you are still new to PHP, then you might as well authenticate the users every time the page is loaded, rather than using sessions and databases.
I am not going to list an entire code because I'm not sure whether you want that. But my tip is to use a file that contains password and is located outside the document root (for added security). Every time the page is loaded, check if the login info has been submitted by the user. If not, show the login form. If so, authenticate his/her password by matching it with the file that contains passwords. Only when there is a match, will you let him/her login.
Again, this is one very simple solution. You can go deeper, depending on how you want/need it.
Hope this gives you some idea. Good luck.