This is a very good question, and to tell you the truth, I too wondered if I could learn reiki online when I first heard about reiki. I did go on the internet to search, although there was not as much information 13 years ago on the internet as there is now.
There is a lot of useful information on the internet, but there is quite a lot of misinformation too, especially about the meaning of different reiki symbols and their uses, which by the way, are supposed to be secret and should not be publicized.
Because I know the benefits of learning reiki from an experienced teacher who can give you in-depth explanations and answer your questions, I strongly recommend taking a course from a recognized and experienced teacher in a live seminar, and not online.
During a seminar, you will learn the history of reiki, different uses of reiki, how to send distance healing, practice how to sense the “ki” (energy), and you will receive attunements.
We all have the ability to receive and share healing energy, but by “attuning” ourselves, we can facilitate that flow of energy even better. Attunement, for the lack of better word, is like a ceremony but most people feel the difference before and afterward.
At least for me, until I received my first attunement, my hands always felt cold even in the summer. But after the attunement, the palms of my hands always feel like I have another thin layer that’s always warm.
Some say that in this time and age we should be able to learn anything online, and I don’t disregard the benefits of online learning, but I also know its limits. So if there is a course nearby, I would strongly recommend to attend a full seminar and learn from a teacher, who has good reputation, and whose fees are reasonable.