In Japanese manga and anime, Ki is depicted as an almost psionic power; with it, you can sense your opponents without seeing them, and you can blow them away with a Ki blast. Real-life masters of energy can do some pretty amazing things with it, though what they do with it is far less visually exciting than anything in a flashy cartoon (I mean, "Dragonball Z" is FANTASY). However, entering the realm of energy in real life is like opening a whole new can of worms.
Aikido ki, in my opinion, is nothing more or less than love in action (read this link for more).
Playing with energy for the sake of learning how to do cool attacks, or for adventure or excitement, is like playing with fire with an open can of gasoline nearby. If you do not have compassion (love, charity) as your absolute topmost goal, then do not start looking for ANY kind of power, especially this type.
Entering into the realm of non-physical energy with a selfish attitude (whether you want to do ki blasts, or you want powerful weapons, or want to use an ouija board or whatever) is very much like oozing blood in a sea of invisible but hungry sharks and parasites. The sharks and parasites are out there. Maybe you've felt them -- that little mean streak, that selfish urge, that depressing influence, that smoldering resentment, that sudden impulse to do violence or cruelty. YOU DON'T WANT THEM. YOU DON'T NEED THEM. Trust me. One's own unhappiness and selfishness are hard enough by themselves. Don't attract even more and make things worse.
You have a choice. (1) You can embark on a journey where energy is used in power games for showing off and making yourself look good, or perhaps for just satisfying your own curiosity. (Yes, even plain curiosity is a dangerous reason to be playing with fire.) Realize those you will run into on this type of journey will have similar goals, and that you will soon enter into a power contest, or into a realm in which people will exploit your curiosity. You will find visible and invisible sharks here, egging on your ego and selfishness and weaknesses. (2) Or, you can embark on the higher, narrower road, where the goal is to heal others, help each other, neutralize strife and lead on towards peace, sometimes using "energy" only if absolutely necessary. Note here: the LOVE is primary, the energy is a far secondary concern; a mere tool. The choice is made in one's own attitude and one's own intentions.
Ki blasts? Sure, some people can eventually get to the point of doing something like a ki blast (again, not the flashy cartoon stuff). But a ki blast is usually brute force, and brute force is the last resort of the incompetent. Fortunately or unfortunately, real love in action would choose to neutralize suffering before forceful action ever becomes necessary. Real aikido would rescue the unhappy would-be perpetrator of crime BEFORE he commits a crime. It makes for what would be a visually 'boring' cartoon, but how much better it is to relieve suffering directly than to have to beat down an opponent driven by hatred and despair!
In my aikido training, I learned that without peace and compassion, "Ki" is NOT Ki.
I could go on into the esoterics of the spiritual realm, but I'm not sure many people would believe me. There is a definite reason for many religious prohibitions against playing with psychic forces. But I will say again: experimenting with energy for selfish fun ... or worse yet, for the purpose of hurting or threatening others ... will call negative influences upon yourself. Sometimes the result is immediate and obvious; sometimes it is slow, insiduous, and subtle. But rest assured you get back what you put out.
Too often people forget or don't mention how important attitude is. Ki in aikido incorporates the idea of a relaxed, positive, giving, compassionate mind. Again: without the peace and compassion, Ki is not Ki. If there is strain and forcing and brute strength involved, it is not Ki!
And without Ki, an aikidoka is out of tune with the universe, and must rely on his or her own body's strength and/or energy -- which is finite. With Ki, the aikidoka is more in tune with the infinite universe, and as a result is far stronger, more centered, more relaxed, and far more immune to attack -- and far more likely to be able to help others. This is why aikido done right is so powerful for even small or weak people; it is tapping into the power of the universe. Or as I see it, the power of God.
Good Ki can be seen all around us: in a powerful painting; beautiful music played by someone who can really convey it; or even in a friendly smile of someone who is willing to listen to others' problems. Better Ki is to be found in a person who, out of his or her own inner compassion and peace, reaches out to help others ... far more than is to be found in a martial artist who can chop blocks in half but who doesn't do a darn thing for anyone else. Which person helps more people? Which one brings more peace and healing to the world? Which one is more powerful?
If you still insist on wanting to learn about energy, you might try starting with the martial arts. (You may also want to read some books about aikido or budo/martial arts philosophy, such as this selection at Round Earth Publishing.) While most do not teach about energy explicitly, the discipline and awareness you build may help you to become aware of and work with your own energy. And of the martial arts, I personally would go with Aikido (I'm biased for the Ki Society), tai chi, or possibly yoga. Yes, these are "softer," but my personal sense is that it is too easy to fall into an aggressive mentality with harder martial arts, and from what I know of aikido, aggression is seen as being out of tune with the universe. Aikido cultivates the "non-fighting mind."
Make sure you find a good teacher, one who is patient, kind, and with whom you feel comfortable. The style of martial art matters less than the quality of your teacher. Occasionally even a good dojo may have a bad teacher, or a bad dojo may have a good teacher... don't give up; look for someone who lives by the highest principles you know.
Other seekers may try looking into certain types of religious/spiritual training. If you have wondered about some of the miracles performed in holy texts throughout the world ... you can rest assured some of them were really done and involved a lot of non-physical energy. But as with anything else, any religious training that stresses the self over selflessness, aggression over peace, hate over love, is just another trip into the nasty swamp. Here, too, finding a good teacher is often more important than a particular belief system.
And don't forget: truly powerful and positive music, art, literature, speeches, and even business leadership all contain Ki. The martial arts are not the only path to Ki. Anything that is good and true will contain Ki, because real Ki is a natural part of the human spirit at its best.
Seek peace over violence; love over hate; truth over deception.
Remember your goal, your true inner direction, must be toward compassion, peace, and truth. If you "seek first the Kingdom of God," all else will follow. If you seek first yourself and your own gratification... watch out.
And lastly (from another of my web pages):
One of the simplest and best examples I know of Real Ki is the extension of good will that is an honest smile or a small expression of caring. Just think -- one smile, or a few kind words, can positively change the day for another person, and possibly even cause a chain reaction of good will, like the "Ripple Effect." Now, for something so small to affect so much -- isn't that true power?