There are several ways to understand this question, and I will provide the answers to all of the following potential meanings:
As far as how to learn Reiki, I would most definitely recommend taking an in-person course, as Reiki cannot be learned in any other way. You can certainly find a lot of information about the history of Reiki online or by reading Reiki books, which may give you a good starting point; however, because of the nature of this Japanese healing art and because of how it is taught, it is impossible to learn it online or from reading a book. There are several inherent processes to learning Reiki that must be done in person and cannot be skipped. If those essential elements are dropped, you may find that the technique you learn is not very effective, and truthfully, it cannot be called Reiki at that point, as it will no longer be the true authentic Reiki in the way it was originally taught and practiced, but a different healing art (similar to Reiki, but different). So, taking an in-person Reiki course is the best way to learn Reiki.
Finally, if you are inquiring about getting a course of Reiki sessions, then it is definitely a great idea if you have any chronic conditions or serious concerns (as opposed to new, and/or minor symptoms). Your Reiki practitioner will be able to recommend the duration of the Reiki course. Think of it the way you would think of a course of antibiotics or of a course of physical therapy. One pill or one session can help, but it may not be enough. Ideally Reiki should be done daily or nearly daily, until the problem is resolved. In my experience, however, it may often take just one session or only a few sessions to address various wellness concerns, often to my disbelief as a medical doctor (we do not understand how Reiki works from the medical perspective, and in all honesty, the effectiveness of Reiki is often shocking to me, since medically-speaking, it should not work, but it does). Getting a course of Reiki sessions can be very useful, especially when trying to address long-standing, serious concerns.