Things you need to start: a notebook, a pen, and a read of this website. Supplies aren't necessary - there's little point until you work out if this system is for you. So starting off with a crappy notebook (or even some paper you have lying around) and a regular pen is probably a good plan.

If you wanna go in, the Leuchturm 1917 dotted notebook is the notebook most people use, I think. That, a nice pen, and a ruler will get you pretty much anywhere - and maybe some coloured pens if you want to add a splash of colour (or lots of colour).

Over on r/bulletjournal, you'll see people with absolute buttloads of supplies. None of that is necessary - but if you get into it as a hobby or a mindfulness thing, as seems to be popular there, then they might be nice. Personally, my bujo is more of a functional-only thing, so I have no need for that stuff, but to each their own.

All you need is a notebook and pen! It can be any notebook and pen, as well. If you want to be fancy, a lot of people recommend a Leuchtturm1917 notebook. You should also read about the bullet journal method.

But the most important thing is to have fun and use this journal to organize anything you need in your life!