Welcome!
1) You can change the files with any editor and run them on your local computer or your web host. You will want an editor with at least "syntax-highlightning". I recommend setting up the webserver and database on your computer, instead of having to upload the files each time you change them.
2) You decide 🙂 Usually people group them by "concepts", that is, all user-related stuff goes to a single file.
3) Depends on your database backend! Mysql, one of the most popular, doesn't have a "path in php" per se. You install both, and PHP knows what stuff to call on your computer to access the Mysql data.
4) Just open <?php, close ?> and write code in the middle :p PHP can be inserted into HTML. In a few months you will learn this is a bad idea 🙂 to mix "business" logic with presentation logic. But it makes starting very easy.
5) You can use any DB that PHP has a backend for it, a module, a way to connect. Check the manua _