Practice your forehand technique by having a coach or friend throw a tennis ball over the net toward your forehand side. Give yourself enough space by standing halfway between the service line and the net. Let the tennis ball fall below net height before hitting it. Aim your forehand shot to clear the net and land within the opponent's baseline. Your technique will develop as you learn to generate enough topspin to get it over the net and land in bounds.
Improve your backhand technique with a drill similar to the above forehand drill, except that your coach needs to throw the tennis ball over the net toward your backhand side. Hit the ball with a strong backhand shot once it clears the net height. Increasing your swing speed will make the drill more difficult as you have to maintain control while creating a large amount topspin.
Sharpen your hand and eye coordination by playing against a wall. Change which hand holds the tennis racket between shots. Standing closer to the wall makes the exercise more difficult as it requires faster reflexes and more hand and eye coordination.
Improve your serve by placing three tennis balls spread out in the opposite service court and then attempting to hit them one by one with your serves. Switch sides once you've hit all three.
Work on your general athletics. You don't need to lift weights to make a tennis team, but you do need conditioning and endurance. Jog for a half hour each morning and jump rope. Swimming is another good exercise that works your entire body.
Improve your hand and eye speed by quickly grabbing a tennis ball that a friend drops from his hand. Repeat the exercise until you can consistently catch the ball before it hits the ground.
Play practices tennis matches with friends every other day if possible. This will work all aspects of your tennis game and keep you in the competitive spirit.