You might want to look into an edx course. Just a little over a week the W3C HTML5 course started.

I don't know if it's the best way on how to get ideas, but they will have (weekly) tasks you have to do and I think it's a good starting point. The C# course I did a couple of weeks ago had you build little applications as bonus tasks, for example little calculators. Currently there's no proper web development course scheduled there, but I'd keep an eye on it, I'm sure a lot of people are wanting that.

Also, keep in mind that HTML and CSS alone won't really get you anywhere. I'd suggest you peek into PHP, since it's pretty easy readable and in my opinion easy to understand and you can see the results right away. Also, PHP will make clear, how you do logic/data in PHP, display stuff in HTML and CSS and how they're all connected, yet different layers. There are also a looooot of PHP examples and guides. People who build voting websites with it as a guide, and many other things. You can relatively easy and quickly see results with it. Building a small and simple quiz website with a proper tutorial can be done within a weekend, I'm sure.

And it's always good, as the other people have stated, you will need an idea first, and that will make things way easier. Something is annoying and repetitive? Why not develop something to automate or replace it. Maybe you're not happy with the apps and tools for your private notes. Write a small website where you can store your notes and manage them however you want. You will need to take small steps and it will take a lot of time. Start simple, think big. Tackle one problem at the time. No matter how experienced you are, you will have to review and review and review your code agian and again and each time you will learn a little bit more about coding.

Last but not least, a small starting point for you: The website pluralsight is awesome! I quickly checked and they have a couple of useful PHP tutorials. One tutorial is called PHP: Getting started, and that might be a good way to start. Another one starts diving deeper in and will use PHP and MySQL to build a dynamic website. You should be able to use the free trial for both those tutorials, otherwise it's money well-spent, their tutorials are awesome! Otherwise, google will be your best friend for PHP. You will find looooots of help and guides/tutorials online.

Good luck!