I'm getting blurry photos

I have a nikon D3100 and i' using 55-200 with a Yongnuo YN560-III Speedlite, i don't know why i'm not getting sharp photos indoors,?

Sometimes a fuzzy photo can be when you are not holding the camera still. When you take a picture try holding the camera still for 3 seconds after the shutter has closed. It could also be a matter of bad lighting and the camera does not agree with the light. Try the flash or no flash. You could also try using a tripod or gorilla pod for better results.

Are they "blurry" as in not focused, or are they "blurry" because of motion blur/camera shake?
If the former, there may not be enough light for the camera's autofocus to work properly, even though when you take the shot the flash provides enough light for proper exposure. Unlike dedicated Nikon flashes (or some other 3rd-party flashes for Nikon), the flash you're using doesn't do a TTL pre-flash or project a focus-assist beam, so that might be the problem.

There's usually a choice for auto focusing or manual focusing If you keep it on auto and accidentally hit it onto manual you can get blurry pictures that way. There's a thing on that camera called back button focusing here is the tutorial.

another tutorial http://www.dummies.com/how-to/content/how-to-choose-a-focusing-target-with-a-nikon-d3100.html
http://www.cameralabs.com/reviews/Nikon_D3100/lens.shtml