Guidance | If you're just starting out then bullet journalling can seem daunting. There's a lot to get your head around and there's no way you can get started without a basic understanding of the system and how everything works. I'd recommend starting here to get in a idea of what it's all about and then heading over to Boho Berry for a more practical look at how the bullet journal can be used..
A notebook | No brainer, right? The beauty of the bullet journal is that you can use any notebook and don't necessarily have to face any upfront cost just to get started. A dotted Leuchtturm or Moleskine tends to be the norm but you definitely don't have to stick with either of these. Personally, I like the clean look of the dotted Leuchtturm which makes it my notebook of choice; definitely order through Amazon (where they tend to be cheaper) and there's also the official bullet journal version available.
Pens | Another obvious one; you're going to want pens that aren't going to bleed through the page and spoil all the effort you've gone to in getting your notes looking nice. I use these for the basis of all of my journalling and then have a couple of coloured pens for accents. A good place to start is with Staedtler Triplus Fineliner and the Tombow Brush Pens are great for bulk washes of colour. And so many shades to choose from, too!
Things that need organising | A bullet journal isn't going to be for you if you don't have any ideas about things you want to track or organise as this is definitely where the fun kicks in. Nobody needs another half-used notebook sat in their collection and if you have nothing to journal then there's no point in even considering starting. Monthly and daily to do lists are a given, but if you're struggling beyond this then check out this post for more inspiration.
Time | I don't think I realised when I first started quite how much time bullet journalling can take up! Mapping out and drawing spreads can easily eat up an hour or so, so be prepared to invest your time if you want everything to look just so. Thankfully I've found a more minimal (and quick!) style of layout which is both pleasing aesthetically and suits all of my needs.