Cutting Mats - Protect your table.
The next thing that you might want to invest in is a good cutting mat. While these aren't completely necessary, they're cheap, and make your life much much easier. I've gone without a cutting mat for a few projects, and instead have used a stack of newspapers or a magazine instead, and while that works and is free, you end up chopping the newspaper into tons of little pieces that gets really annoying to clean up. You definitely don't want to cut directly on top any surface you care about, because those hobby knives will completely destroy whatever is underneath. So spare yourself the pain and pick up a cheap self-healing cutting mat.
The main thing you need to think about when getting a cutting mat is what size you're going to want to buy. For most papercraft projects, you'll be printing on letter sized 8.5x11" paper or A4 paper. This means that a 12" x 18" mat will be sufficient surface area to work on. I use this size mat for all my projects and haven't looked back. Below are some suggestions of cutting mats you might want to invest in.