Test the keyboard and mouse by opening some programs with your mouse and typing some text with your keyboard. It's a good idea to test every key to ensure there were no issues during the manufacture of your new keyboard.
If the keyboard and/or mouse do not function, check to make sure there is no interference and that the equipment is in range of the receiver. Also, check the troubleshooting information probably included with your manufacturer instructions.
Remove the old keyboard and mouse from the computer if they are still connected.
If you plan on disposing of your old equipment, check with your local electronics store for recycling information. If your keyboard or mouse is Dell-branded, they offer a completely free recycling service by mail (yes, Dell covers the postage) that I highly recommend you take advantage of.