After lettering for about a year, I've had time to experiment with a handful of supplies and I now have a list of go-to items I can't work without:
Moleskin notebook & scratch paper - I love the simple & functional moleskin notebooks and I use this one for all my personal lettering projects so they're all in the same place. I also love using scratch paper to work out thumbnails and rough sketches before moving to a full size drawing.
Compass & ruler
HB pencil & kneadable eraser
Micron pens in various sizes - these are by far my favorite pens for all outlining and drawing. Sharpie pens are a close second, but Microns have such great line quality in all different size tips.
Faber Castell Artist pens in various sizes - these pens are great in the brush, bullet, and giant size tips for filling in large areas and brush work.
I'd highly recommend these supplies if you'd like to experiment in lettering, but remember that tools don't make the artist - start with what you have to get a feel for drawing letters before spending lots of money on new supplies.