Research, Research, Research
Sometimes it is said that "half of all writing is re-writing". If that proportion is correct (and I think it's a low estimate, myself), then most of the other half is research.
You said it yourself, you don't know much about amnesia. That's actually good! Now you get to learn all you can about amnesia. This is one of the fun parts about being a writer! We get to learn enough to be dangerous/convincing about all kinds of fascinating topics.
The web search
The easiest way to get started is to start searching the Internet. Obviously, first stop is Wikipedia. While you're there, look at the cited sources at the bottom of the page. If there are other web sites cited there, then you can easily go to those to learn more. But don't limit yourself to just web sites. Go through any book references and see which ones are available digitally, or in an affordable paperback format. Buy those if you've got some money available. If not...
They might seem like relics from a bygone era, but there are still plenty of resources that are gathered in libraries that aren't necessarily available online. At the very least, part of your web research should include a search of your local library's catalog on the subject.
All along, keep an eye out for two things:
- References for other materials to track down
- Surprising facts and anecdotes about the subject that can lead to new story ideas
Finally, if you can swing it, work to achieve the holy grail of all research:
An in-person interview with someone who will allow you to record it and is a doctor specializing in memory loss or a patient who has suffered memory loss will be the best possible thing you can do to prepare for your story. Ideally, you would interview at least one of each, and you might be able to get referrals from one to the other.
Some important points about interviewing:
- Be respectful of the interviewer's time and their story - Giving you an interview is time the interviewee is spending away from their work and their family and friends. Ask them for an appointment, don't push if they don't want to meet, show up on time, end the interview on time, come prepared with questions, allow them to not answer, etc.
- Try to have the subject do as much talking as possible. Start with a few yes/no questions about things that you really feel like you need to know to write your story, then ask open ended questions that will hopefully encourage them to talk about things that you wouldn't even think to ask about.
- If the subject doesn't mind, record and take notes. That might seem redundant, but really you want to be writing your thoughts and impressions, and the recording is a backup and fail safe. It also allows helps you not have to ask the same question twice because you weren't paying attention to the first answer - you can just go back later and listen for anything you missed.
- Establish and respect any anonymity requested by the subject. In your particular case, it's likely illegal for a doctor to give you the names of any patients, but you can give the doctor your contact info and ask them if they are willing to talk to patients about reaching out to you. If you're telling a positive story, some patients may be very happy to have their experience told to a wider audience.
- Offer to thank all subjects in your acknowledgements, and then follow through on their preference. Sending thank you notes and even small gifts after the interview isn't a bad idea, depending on your culture. Assume you will want to interview all of your subjects again and treat them as kindly as possible.
If you can't find anyone who will agree to an interview, that's ok. Interviews are wonderful types of research, the ultimate, in my opinion, but you can learn a lot and write well without them.