Play with the settings and look closely at the photos. Are you off on the focus - ie is there somewhere that is in focus, just not where you want it? or is there nowhere really in sharp focus. The suggestions above are good too. I am sure there are plenty of tutorials on the web talking about focus for portraits. Try a tripod and mirror lock up and timer and live view zoomed in focus on a static subject to see what the lens/camera is capable of.
You can make things look sharper with various sharpening in post or in camera, but you should first find the sweet spot settings for your lens/camera and start with the best image you can get.