It is necessary to disinfect a toothbrush after using? if so, what is the easiest way to do this?

Not really: Once you are done brushing your teeth rinse thoroughly and let air dry. Your toothbrush should not be closer than 6 feet from your bathroom toilet. There is some growing evidence that flushing your toilet can contaminate your toothbrush at this distance or less. Soaking in listerine or any disinfecting mouth wash may help. The key is to use a soft toothbrush for no longer than 3 months.

It is not necessary: There are a host of gadgets available if you so choose. Just apply common sense - rinse the toothbrush well after use, leave it out to dry; replace frequently, replace if you have just recovered from the flu, cold etc. Hope this helps.