What Causes Pimples On Face And Upper Body?

Hi,

I suddenly had boils/pimple on my upper chest, shoulders & face about 3 weeks ago. Some of them had pus. They lasted about a week and went away but left dark scars. Now, I am have got the pimples/boils on the upper torso and face and some of them have pus. What is the reason & how to treat it?

Thank you.