I think if you have the basic supplies, you may need some good white cardstock to print your digis on. What colouring tools do you have? Pencil Crayons? Copics? Tombow? It will depend on how you're going to colour the image as to which paper to get. Also, if you don't have anything to colour with, I'd start off with a set of pencil crayons. I started off with Crayola and still use them although I have added Prisma and Koh-i-noor to my repertoire as well.
You may want to look through the galleries too to see what you're drawn to and see what materials are used. This will give you an idea of what you will need first. I say first, because there's always something else we need.