Why do pictures I take with my Nikon D3000 come out blurry?

I don't have vision problems and I don't suck as a photographer, but every photo I take without Autofocus comes out blurry. I don't like using auto but if I don't my pictures are always slightly off. They look great small, but then I blow them up and it looks slightly blurry, making them unable to be enlarged without looking bad.

I tested it and when I use auto focus, my subject looks slightly blurry, but the photo comes out clear. What could be causing this and what steps could I go through to fix it? Is it worth repair or should I buy a new camera/lens and how can I troubleshoot on which to purchase?

Please help!
>>> Why do pictures I take with my Nikon D3000 come out blurry?