It's hard to give advice without knowing exactly what you're trying to do, but it sounds like you'd be better off customizing an existing CMS than rolling your own. Joomla is the most obvious choice for you, it's well-documented, has a large community, and is written in the language you know. You could probably kludge something together with a passing acquaintance with PHP and MySQL, but you don't want to. Writing software to cough up some text is easy, writing software that doesn't suck to cough up some text is hard.

You can use aws free for a year. After that what it costs will depend on what you need which, after a year, you should know.

In general, PHP written by inexperienced programmers turns into a mess. It's easy to get something working, but very hard to get something you'll be able to read a year later. Try to keep future you and how much you want him to not hate past you in mind.

You will not have thousands of users in the first weeks. That is not a thing that happens. It is a thing that used to happen, when the web was smaller, but it is not a thing that happens now. Get your site working, get some content, and then (and only then) post to relevant subreddits/wherever people interested in what you offer hang out about it.