There are basically seven different directional placements in table tennis, though only five or six are available at any time. They are:
Generally, you never want to put a ball to the middle forehand or middle backhand, where your opponent is just standing there, ready to hit a forehand or backhand. Most often you should be going to the wide corners or the opponent's elbow. The other options are outside the corners, but these can be tricky to play into - you have less table, and so it's easy to go long (or more specifically, off the side). Here are your options for going outside the wide corners:
When you can play into those wide angles outside the corners, your opponent will have great difficulty as you greatly increase the amount of table he has to cover. So add these angles to your repertoire and turn table tennis into a truly full-court sport.