Assuming you're referring to the area beneath the mower deck where the blade is located:
The first thing you'll want to do is disconnect the spark plug wire so that the engine can't accidentally start while you're working. Once that's done, tip the mower onto its side so that you can access the underside of the deck. Use a putty knife or similar tool to scrape any built-up grass clippings and debris from under the deck. If there's a lot of buildup, you may need to use a power washer or hose to blast it clean.
Once all the visible debris has been removed, it's time to disinfectant the area to prevent mold and mildew growth. A solution of 1 part bleach to 3 parts water will work for this. Apply it with a sponge or brush, making sure to get into all nooks and crannies. Rinse thoroughly with clean water when you're done and allow everything to dry completely before putting the lawnmower back together.