Believe it or not, Legally, you really cant do anything. But if you contact your local wildlife or fish and game dept, they can give you the numbers of your local wildlife rehabbers, Veterinarians will also have these numbers, and some short term instructions. The milk is really specific, and some rabbits will die from cow milk, so make some calls quick, in the mean time, keep the poor lil thing in a WARM and dark soft nest, shoe box with towel and a heating pad on low will work. Put the heating pad under one side of the box, and let the critter move to the temp area that it wants. Resist letting people look in on the baby. One look every few hours is a max.
And watch your cat, He/she knows where that nest is, and will go back for another toy. Dont put the baby back, mom wont take it. but keep kitty in, or he will be bringing you more for your collection!
Make calls, and good luck!