Watercolor Paper: Canson XL Cold Press size 9x12 is a great option. Don't go under 140 lb/300g (you will see this info on the bottom of the cover). The 9x12 is a great size because you can cut it in half to make two cards or just leave as is for a nice sized piece of art. When you are ready to move on to a more expensive, higher quality paper, I highly suggest Arches Cold Press.
Kneaded Eraser: This looks like a little grey square in clear wrapping. It will be by the art pencils. Prismacolor or Faber-Castell are both great brands.
Plastic Board: I just bought a corrugated plastic poster boardand cut it down. You can find these in the poster board section of your store. You can also just tape your paper down to your working table if you need to. And just FYI, I only ever tape down my larger paintings that have a lot of water. If you are using a 140lb paper like I mentioned above, and just doing a card or small painting, I wouldn't worry about taping it down.
Light Board: My favorite light board is my window but if I'm working at night, I either use theSoftbox appon my iPad or my trusty TickTek LED lightboard. I like this lightboard because it is very skinny and easy to store.