Kicks often lead inexperienced martial artists open for an easy attack. The most practical kick in a self-defense situation is the Stomp Kick and the Foot Stomp. These kicks do not require much flexibility and are usually less obvious to an attacker.
Pick up your knee. Then stomp in the direction of your target. You want to strike with either the inside or outside edge of your foot. The knees are your #1 target for this technique. When you strike them you want to force them sideways, then down to the floor.
You don't need to lift your knee as high as you do for the stomp kick because your target is on the ground. Lift your knee. Then stomp on your opponent's foot using your heal. Put all your weight into it. A simple stomp can send your opponent groveling to the mat.
If you still want to learn other martial arts kicks to add to your self-defense tool box, here's some more information:
The two best ways to improve your kicks. Stretching the hamstrings, quadriceps, groin region, calves, gluteus maximus and lower back all increase your control and range of motion for kicking. When you work out, you should always stretch before and after. Stretching before loosens you up for your workout and helps avoid injuries due to hyper-extension. Stretching after your workout is the best way to improve your flexibility and vital if you want to improve your kicks.
The front snap kick is directed forward toward the opponent's stomach, sternum or crotch, and with practice toward the face. Without shoes the striking surface is the ball of the foot. However, with shoes on, which is the more likely state you'll be in when using self-defense, you should kick with the toe of the shoe if it has enough support. You begin the kick by lifting the knee directly in front of you. Then extend the foot to strike. To speed up this kick, concentrate on lifting your knee as fast as you can. This will naturally increase the speed at which your foot extends toward the opponent.
This kick requires you to pivot on your back foot so that your planted foot points backwards. As you pivot you point your butt toward the opponent while raising your other leg and swinging your foot across the face of your opponent. When wearing protective shoes, you strike with the top of your foot. Martial artists may strike with the ball of their foot, but this is very difficult without proper training and instruction.
This kick is used usually for someone behind you. You simply lift the leg farthest from the target (your front leg), and kick back with the heal of the foot. This kick is extremely powerful. However it is more difficult to aim and should be practiced with a heavy bag or other target.
Side Kick This is much like the back kick except its aimed toward your side. You must pivot, as you do for a round house kick, so that your planted foot is pointed away from the opponent. Lift your knee, and strike with the outer edge of the foot and heal. As you kick rotate your hips for added power.
It's also possible to spin when using these kicks.
This kick is effective when you are extremely close to the attacker as when the attacker is grabbing or holding you. Grab the attacker's upper body and pull him in to you. This decreases the attacker's mobility and downward visibility. Then strike the outside of the attacker's knee with your knee or shin. The outside of the attacker's knee is a very vulnerable spot. You can also stomp on his foot, knee him in the crotch or eagle claw his sides while in this position.