The World Health Organisation has advised the following on the use of face masks. More information is available here.
If you are healthy, you only need to wear a mask if you are taking care of a person suspected of having 2019-nCoV infection.
Wear a mask if you are coughing or sneezing
Masks are effective only when used in combination with frequent hand-cleaning with alcohol-based hand rub, or soap and water.
If you wear a mask, then you must know how to use it and dispose of it properly.