Steps
Warm up - (10 minutes) running in place or around the block for about 5 minutes; plus about 5 minutes (or 20 reps each) of push-ups, sit-ups (or crunches), leg lifts, and reverse push-ups.
Stretch - (15 minutes) stretching all major muscle groups first is essential for a loose and limber body; get a book on stretching if you do not already know what stretching exercises to do. In Karate, stretching the legs are crucial to keep yourself injury-free.
Meditate - (5+ minutes) clear your mind of all thoughts; concentrate on breathing in through the nose, out through the mouth; steady deep breaths and a clear mind will prepare you to learn Karate. There is no time limit, but meditating for at least 5 minutes should clear your thoughts enough to allow you to concentrate; forget school, work, family, problems, everything -- visualize them evaporating before your eyes... once everything is gone, you should see an empty room, and in the center of the empty room, a ball of flame begins growing from the emptiness... this flame of firing strength and energy should represent anything you hope to achieve by training yourself in Karate... by the time you are done with your meditation, the room should be entirely engulfed by nothing but the flame.
Punching/Blocking (15 minutes) - There are a few essential punches you will need to learn to attack effectively. The straight punch, upper-cut, knife-hand, spear-hand, elbow strike, backfist. Practice them in order and alternate hands. Practice blocking as though these were the punches you were being attacked with. Experiment with combinations and counter attacks. Defend, attack, defend, attack...etc.
Kicking (15 minutes) - ten repetitions of any kick will suffice in strengthening your legs. Focus beyond the target for maximum power, but practice the flow of motion to gain graceful fluidity in your movements; like a swan; power will follow.
Sparring (15+ minutes) - Find someone to fight, and use all of your techniques to fight them for 15 to 30 minutes. Sparring will help you increase your stamina and ability to throw combinations and defend yourself against multiple attacks or multiple attackers, once you've mastered certain blocking and attacking techniques.

Tips
When punching, stay relaxed until just before you hit your target. Bruce Lee says, "Relaxation is essential for faster and more powerful punching. Let your lead punch shoot out loosely and easily; do not tighten up or clench your fist until the moment of impact. All punches should end with a snap several inches behind the target. Thus, you punch through the opponent instead of at him."
When kicking: straight kicks may use the ball of the foot or the heel, keep your toes back or they might be damaged; side kicks always use the blade of your foot or the heel; ax kicks always strike with the heel (but this is an uncommon kick for most); round-house kicks may strike with the shin, top or ball of the foot. Round kicks with the shin may be particularly effective.
When stretching: stretch everything; even your neck, back, stomach, arms and hands... concentrate especially on your largest muscles - the legs.
When sparring: when attacked - 1. Strike them before they strike you; this may negate or impair their attack, cause them damage and expend a minimum of your energy. 2. If this is not possible, move, change the distance and/or move off the line of attack ready for a counter-attack. 3. Block. Blocking need not occur with the hands and it is very unwise to block medium to low kicks with the hands. this leaves the head unguarded and vulnerable. Blocking kicks with the hands (especially open hands is not smart unless you are an expert). The best defense is not being where you can be hit.
If you are interested in martial arts, go and train and stop reading Wikipedia!
When attacking - look for your opponent's weaknesses and strengths. Give your opponent a false sense of security by faking an attack they will use their momentum to block, then attack them using your own power and cunning to lay your opponent out for the count...
You may want to try meditating at the beginning of the workout. This way your mind will be clear and ready to workout without getting your body warmed up and then have it cool down while you sit motionless meditating.
Never under estimate, or over estimate an opponent. The more you're sure you can defeat/be defeated by them, the less/more likely that outcome will occur.
Practice everything you know so much that when a real fight comes you don't have to think, just do. Use a training dummy after every warmup to fight with...

Warnings
Always stretch, and stretch everything. Pulling a muscle or snapping a tendon is excruciatingly painful!
When blocking a kick or punch from a sparring opponent, always remember to clench your fist tightly or you may break your own hand on their kick.
Remember your sparring partner is real, not a punching bag. Safety gear is advised and always be careful.