When it comes to strumming, the best bet is to listen to examples and try yourself to achieve the exact sound to the best of your ability. When you are playing just one chord, if you use a Guitar Plectrum (a pick) then you will probably achieve the cleanest result. 

There are many different styles in strumming and for more in depth explanations please visit the links page. For a beginner, you just need to be able to ring out nice clean notes, which not only requires a good clean ring of the chord with your right hand, but also your left hand, the fingers have to have a good solid contact with the fret board.