How to create web page, when you know basics?

for a long time I've been learning a lot about creating parts of the web page. HTML, CSS, Sass, JavaScript, PHP, SQL, etc.

And now here I am, standing in the middle of nowhere... I have to create web page, but I have no idea how to do that... What should I start with? How am I supposed to create log-on system? I have so many questions and problems but I am not sure how should I solve them.

I would be glad if you have any web page files to learn how it is built. I mean with php files and sql database and some pre-compiled files.

For instance, how am I supposed to check if user is logged on? I think it should be done somehow by comparing some data in session and database.

Please, don't tell me, that if I have THAT type of problem I don't know enough. That's why I am writing this question.

PS. I am self-taught.