You may not notice if your shot is slightly out of focus when you are filming, but it really shows up later when you come to editing. A little extra work can keep our pictures sharp every time.
Video cameras usually come with an auto-focus feature. However this relies on being able to see 'edges' and contrast in the image to be able to work out how to focus. If you are in a low light situation, such as indoors, try zooming in close up and picking out the eye of the person being filmed. This has lots of contrast and edges for the autofocus to pick up. Once the camera has focused correctly, zoom out again to frame your shot.