Assuming you have halogen headlights and not HID/Xenons, the bulb is an H7. As Shumicat stated, instructions are in the owner's manual. If you don't have one, you can download a PDF copy here from MBUSA: 2001 C-Class Owner's Manual
FYI, replacing the bulb isn't "easy" but it's not really that difficult. The hardest part is that you really can't see what you are doing. You may find that on the left (US driver's) side, removing the filler neck for the washer fluid reservoir helps (one screw holds it to the radiator support, then you pull it out of its grommet on the reservoir). Oh, and you may as well replace them in pairs to retain even lighting. Besides, if you don't the other side will go out very soon, I promise!
After you remove the plastic cover on the headlamp assembly, you unplug the wiring harness from the back of the bulb (look at the new bulb to see how it attaches to the conductors) and then you have to release the spring clip holding the bulb in place. It's sort of like opening a safety pin. The clip is permanently attached, so no worries about losing it once released. When you put the new bulb in, keep your fingers off of the glass. if you touch it, wife it clean. Oils from your skin will overheat the glass and cause it to break when the bulb is illuminated. When you install the new bulb, it only goes in one way. There is a small tab that fits into a slot on its mount. Make sure you feel for that and rotate the bulb until it seats properly, If not, you the beam pattern will be all wrong. Also, it will be more difficult to close the metal clip that old the bulb in place.