Aikido is one of the best methods of self-defense, and one of the most difficult to master. Learning in person from a sensei (teacher) in a dojo (school) is ideal. But with determination, mindfulness and discipline, the dedicated student of Aikido can begin to learn basic techniques online.
How to Learn Aikido Online
Find a willing and able partner. Because Aikido focuses on self-defense, you need a partner who will simulate attacks. If your partner is as invested in learning Aikido as you are, you will take turns being "uke" (the attacker) and "nage" (the defender).
Find a good website that lists some basic Aikido techniques or offers online classes, like aikidotechnique.net. Try to make sure the site is run by a reputable sensei, and that it offers plenty of pictures, video and text for you to study on each technique.
Find a video on ukemi (falling/ rolling) and practice basic backward and forward rolls. Practice these on your own on a mat after you warm up, and before learning new techniques. Both partners will need to learn ukemi; it is essential to good Aikido.
Start with one technique. Make sure you read the descriptions and watch the video of the technique carefully many times. Practice the technique slowly in front of the computer, stopping before the uke would normally fall. If your practice area is too far from your computer, print out any pictures or text for reference.
Learn the Japanese names for the techniques. This will help you when searching for more examples of the technique you're trying to learn. It is also an essential part of learning Aikido.
Practice regularly, at least two or three times a week for an hour.
Read up on the philosophy behind Aikido. Knowing the ideology behind the moves you're practicing will improve your mindfulness, which in turn improves your physical technique.
Find an online Aikido community. Aikido has a tradition of all levels practicing together, and more advanced students consider it part of their training to help beginners. Websites with comment threads, forums and other interactive spaces will give you the opportunity to ask questions of more senior students.