How to strike the ball
There are two general types of kicks based on the position of the ball, ground kicks and volleys.
Ground kicks
In order to produce a quality kick you'll need balance and composure. The placement of your supporting foot is just as important as your other, kicking foot. To control the height of your shot or pass, be aware of where your supporting foot is, in relation to the ball. By placing your supporting foot in line with the ball, you will achieve power while keeping your kick low. By placing it slightly behind, you will produce a rising or lofted ball. Your upper body also plays a role when kicking the ball. When you lean back, the ball will rise and if you lean forward your kick remain low and hard.
In order to get the maximum power from a strike, the knee of your kicking leg has to be above or in line with the ball at the exact moment you make contact. Follow-through in a sweeping motion in direction of your target. If you have trouble understanding this concept, try landing on your kicking foot after you strike the ball.
Volleys
When striking the ball in the air, proper balance is essential. Therefore, you need to adapt to the path of the ball by making quick adjustments in your footing. You are ready to swing only after you have positioned yourself at a proper distance from the ball. You should not attempt a volley, if you feel that you are reaching out too far or when the ball is too close to your body for a good swing.
Some situations may require jumping and volleying at the same time. Stay composed and concentrate on timing your leap.
How to practice kicking
The best way to practice your technique is against a keeper (or a friend) who consistently serves the ball back to you. If that is rarely an option, then any regular wall, ledge or flat surface can work as a replacement. Beginners should work on kicking dead balls (meaning they must trap it before kicking) while advanced players can practice striking the ball without settling it first. Many variations and drills available, such as aiming at a target on the wall or dribbling (in random directions) before you kick. Being able to strike the ball from any situation will improve all aspects of your game which include kicking, for example passing, finishing, clearing and so on. In order to learn the proper mechanics, practice with as few interruptions as possible. Also, look at better or more experienced players and study their technique.
A friend can be very useful when you're working on your volleys. Ask him to throw the ball at your feet and try to hit it back to his hands. Practicing volleys is slightly more of a hassle when you're alone, but it can be done. Simply toss the ball (or kick it) upwards and strike as it drops down. Stay composed and adjust to it its path. Don't swing desperately if the ball is out of your reach.
Different methods of striking the ball
Inside of the foot (push pass)
Put your supporting foot about 10 cm to the side of the ball with your toes pointed in direction of your target. With your ankle locked at about 90 degrees, bring your kicking leg back and swing through the equator of the ball. Your kicking ankle must be perpendicular to your supporting foot. Contact the ball with the arch of your foot and follow-through in the direction of your target. When well executed, the ball will roll smoothly rather than bounce or skip along the ground.
When planting your supporting foot next to the ball, your toes have to be directed about 15-30 degrees outward from your target. This will let you drive your other foot straight at the target, hitting the ball with the outside of your laces.
Place your supporting foot in line with the ball and toes aimed at your target. Bring your kicking foot back and swing it forward in one swift motion. Don't pause between retracting and extending your leg. Accelerate your foot through the ball, as if it doesn't exist. The toes and the ankle of your kicking foot should be stretched and locked. To keep the shot low, meet the ball at the equator or slighly above it. Lean forward and keep your shoulders over the ball. Strike it with the inside of your shoelaces and follow through in the direction of your target.
Position your supporting foot to the side of the ball with toes aimed at your target or slightly to the side of it. Swing forward with your kicking foot and meet the ball with the upper part of your big toe. "Slice" the ball not in the center but low on the outside, so that a spin will be produced. Follow through in the direction of your swing, not directly towards your target.
If you're kicking the ball with your right foot put your supporting foot on the left side of the ball or vice versa. The toes of your supporting foot should be aimed at your target or slightly to the side, in the direction of the curve. Bring your kicking foot back and swing forward aiming for the inside part of the ball. Again, if you're kicking with your left foot, aim for the right side of the ball. Contact the ball with the outside, lower part of your shoelaces. Remember to follow-through.
Your supporting foot should be planted next to the ball with toes pointed at your target. Quickly thrust your kicking foot back and then forward without moving your thigh too much. Sharply cut the ball from the bottom with the the tip of your toes. This will produce a lofted ball, spinning vertically towards you.
The volley is a kick used when the ball is above ground. In terms of technique, it follows the same principles as the instep drive. Producing a good volley is largely determined by your timing. Concentrate on when to swing rather than where to contact the ball. Don't try to hit the ball really hard. If you strike it at the right time, it will have enough power and you'll get more control over its direction. Make small adjustments in your footing so that you are well balanced prior to actually swinging at it.