You’ll be surprised at how little you’ll need to start programming in Javascript:
1. A computer — that’s the thing you’re reading this on.
2. A web browser — that’s the program you’re reading this with.
3. A text editor — text editors are simple programs, and there are a zillion of them. A text editor is like a word processor without any ability to format your text. You get just plain old everyday text to edit, nothing more. You already have at least one such program on your computer. Windows comes with NotePad and Mac comes with TextEdit. Both of these are adequate to your needs, but very few programmers use these extremely simple text editors. At the other extreme are super-turbocharged-ultra text editors that will edit text in more ways than you can imagine. Professional programmers prefer these gold-plated applications, but you don’t want anything that complicated. Besides, the top-notch text editors cost money. Bah! Humbug!
I won’t recommend any particular text editor for Windows or Linux, as I don’t know them. Just search for “text editor Windows” or “text editor Linux” and pick whatever suits your fancy. Get one of the free text editors; there are plenty of excellent free ones. If you become a professional programmer, then you can get the gold-plated text editor.
For the Mac, I do have a recommendation: BBEdit. I’ve been using BBEdit for 30 years now, and I’ve never had a single problem with it. It has lots and lots of wonderful features that I have never used. There are two versions: a free version and a paid version ($50). The paid version has magnificent features that I’ll never dream of using.
Once you’ve got your text editor, you’re all set to begin programming.