What Causes Pimples With White Heads On The Face?

I had a clear skin before though I have a tan complexion. But now since two years I jad started geting pimples with white heads. Thought it doesnot come on my face all tje time. I get two or three . And the marks of pimples goes only after four months. And I start gettings pimples again in the place where I got before. I even pop it out with my nails sometimes. What am I to do? Recently a doctor has prescribed me for a cream called melaglow and peroclin. Will it work? N will I get clear skin?