I'm a super newb: Where to enter code?

I'm a super newb: Where to enter code?

I've been teaching myself how to code. Now, I want to practice my newfound
coding skills by creating a very, very simple web app. What programs do I
need to actually write and test my code and eventually release the app?
I've been searching around the Internet and keep hearing all this random
stuff about integrating my HTML, CSS, and Javascript code then entering
this or that PHP code to communicate with my MySQL database, etc., etc.,
blah^3. And that's all fine to know...but WHERE does all this code go?
Where am I actually typing all this stuff to then preview it and
eventually release my app?
Please let me know if my question doesn't make sense!