When framing a shot you should ask yourself what and who is the main subject of interest and where the interesting action is and focus on that. Try to aim for variety in shot sizes so that the end product would be lively and breathing and not monotonous. The shot field size means how much of the subject (which often is a person) can be seen in the shot.
Here are the four main shot field sizes (examples from Faux):
Next: lighting the shot.