Sometimes it's difficult to begin. There are millions of possibilities, here are a few:
and think of ways you might begin with that element. For instance, you might look at news stories for plot (or you might talk to relatives about their memories, or you might think about some event in your life that would make an interesting story, or...the possibilities are endless) For character, I find it helps me to have a visual of a character, so I cut up a catalog or magazine to find usable images, then I find a "writing character worksheet" (you can Google to find many) and fill it out. For setting I would start with a postcard or some other image, or begin to research a time and place I find interesting. For theme, it sounds odd, but if I hear about something that makes me angry, I can often figure out a theme that interests me. I've seen other writers begin with a quotation they love.