You'll see three tiny wheels called rollers. Don't use an eraser, pen or pencil, since they can leave debris inside the mouse. After you finish cleaning all three rollers, drop the dry ball back into the mouse and replace the collar. Turn the collar clockwise until it stops. Special kits are available for cleaning the mouse, but most people are content doing it the old-fashioned way as described above.
Keyboards get dirty real quick, too.
If your keyboard looks like a mechanic has been using it, unplug the keyboard from your PC and give it a cleaning. Use glass cleaner or warm water along with a soft cloth. Wet the cloth so it's damp, not soppy and then, clean the dirty areas. Try not to get any liquid directly on the keyboard or underneath the keys. Your computer needs good ventilation and should be located in as dust-free an environment as possible.