Hitting the ball deep on your opponent's court is one of the most important things in tennis.
By doing so you keep the other player on a defensive position and allow yourself to gain control of the point. Eventually he/she will make an error or give you a short ball, and you can be aggressive and end the point.
When going for depth, use the net as your reference point. If your balls are not deep enough, simply clear the net a little more. This is the most effective and simple way to approach depth. Avoid the mistake of aiming for "close to the baseline" or "over the service line". This can get confusing and lead to unforced errors.
You can do this very simple drill with your practice partner:
1) Start a rally by feeding the ball (no serve) and carry it in a cooperative manner. No player is allowed to attempt a winner.
2) If you or your practice partner hits the ball long or wide, the other players scores 1 point.
3) If you or your practice partner hits a ball that lands short, inside the service boxes (before the service line), the other players scores 2 points.
4) If you or your practice partner hits a ball into the net, the other players scores 3 points.
5) First player to score 21 points wins.
Remember, the net is your point of reference. Clear it and you will get the depth you want on your shots.