I would be proud to have taken this shot. Not sure if you were looking for C&C, but the one thing I notice (at this resolution at least) is the forehead and nose appear to be more prominently in focus than anything else. I can't help wonder if the focus was set slightly further back, on the right eye perhaps, if it would add a little something?
I like the chosen focus area. It emphasizes the expression and emotion on the man's face. Sure, it would have worked with a larger DOF but I actually really like this version. ...And it's not often that I enjoy portraits.
Aperture is the f/#'s and it controls how much depth of field is in the picture. DOF = how much of the image is in focus. The higher the f/#, the more will be in focus. However, the higher the number, the more light you need to get proper exposure.
You'll need to read the manual for your camera's model to learn how to change the aperture setting.
Play around with it. It gives you a lot of artistic control.