You may already have materials. Please don't feel that you need to purchase exactly what is on the list below. I have indicated with an asterisk which materials are pretty critical to have. Similar colors of paints, paintbrush sizes, etc. are just fine.

Brushes: It is important to have watercolor, not acrylic or oil, brushes! Real tiny brushes are okay for details but won't be very useful for much else. You need the larger size rounds, but if you have a #6 and a #12 round, for instance, that would work instead of the number 4 and #10.
*1" flat, preferably with a bevelled tip on the handle–It’s important to have this brush
#8 or #10 round (Just make sure you have one or two rounds about this size)
#4 round
a fine brush or a larger brush that will hold a fine point, (I often use a # 4 rigger)
*1.5 or 2" flat (or one of those flat foam rubber paint brushes that you get in hardware stores...they work great for big washes)

Paints: (This is my basic palette but similar colors are fine.)
raw sienna (I like it because it's so transparent)
ultramarine or French ultramarine blue
cobalt or cerulean blue
quinacridone red or violet (or alizarin crimson...not quite as permanent a color)
cadmium red medium
hansa yellow light or cadmium yellow medium
bring a green, or we'll mix our own on site
burnt sienna

Paper: Good paper is essential for getting good results. If you are an experienced watercolor artist, use the type you like and are accustomed to, but make sure it's at least 140 lb. paper. If you are not experienced, I suggest you use 140 lb. D'Arches cold pressed paper. 11" by 14", or 12” by 16”.., up to a half sheet (or thereabouts is the most practical size).
Either bring individual sheets and a board, or bring watercolor blocks. The blocks are really nice because they're pre-stretched and eliminate the need for a board. I recommend bringing one block, or about 15-20 sheets of paper.

sketchpad (any size...can even be just some typing paper stapled together if you don't want to buy one)

Miscellaneous: good-sized plastic water container with lid (not glass). I often bring two...one
to clean my brush in and one for clean water . Lots of clean water is critical for good results!
old tooth brush
dozen paper towels
saran wrap
razor blade
a little bit of salt
cellulose or natural sponge
masking tape or bulldog clips if you aren't bringing a watercolor block
palette
drawing pencils
spray bottle
liquid mask
watercolor board if you're not bringing watercolor blocks
some convenient way to carry all this stuff