How Can A Penis Shaft Pimple With Fluid Build Up Be Treated?

I had what seemed to be a pimple on the shaft of my penis. It came to the point where I needed to pop it because it was painful. Now there is fluid build up around the site. Is this a boil?? An std?? I don't know if I should let it heal on it's own or even what it is. Can you give me any insight on what I should do??