I was successful first with colored pencils. They will be much more comfortable to you as a pencil artist than a paint brush. Use brighter colors than you really want and then use a light cream or white pencil over the top (push hard!) to blend them together, mellow them out, and make really nice shading. It's a great cheat I've used for years! :)
Water colors are nice too and give you "instant gratification" but mistakes are hard to fix! Acrylics are a pain in the butt, in my opinion, and aren't much good for anything except craft projects. If you have the time, oils are nice but take a lifetime to dry, especially if you're in a hurry! Good luck!