You've probably discovered by now that just because your camera has auto-focus, it doesn't mean every photo is going to be tack-sharp. You have to work with your camera to get the best out of. Here's some tips that should help.
Get tack sharp images with these DSLR focusing tips
Get out of the focus Auto Area mode, use single area focus instead
Learn about the 'focus and re-compose' method
When to change the selected focus area instead of using 'focus & compose'
Understanding edge contrast
When to use manual focus, and a great tip for more accuracy