Literature search methodology for non-academics?
It can look daunting but don't be frustrated so quickly. Even academics are intimidated by the vastness of the system as well. The game's rule is to dive into the right partition of the literature to maximize the chance of finding what you want.

Before you start, browse the library website of your school for online resources first. Most institutes have online tutorial guiding students how to use different services. There could already be a workshop or video about what you want to learn.

One particular thing we need to know is that unlike public library which usually has one big catalog, academic literature is largely cataloged by their field of study. For instance, if we are to look for medical articles, PubMed would be a good starting database. For psychology, PsycINFO would be a good database. These databases group all related journals inside, and this can filter out a lot of unrelated articles.

Once the right database has been identified. Learn more about it by signing up for an account. Usually each of these databases has a tutorial and also an alert service for newly released articles. For example, this article talks about how to more effectively search PsycINFO.

For PubMed we have something called MeSH (Medical Subject Headings) which guides the use of keyword. For Psychology an equivalence is the Thesaurus of Psychological Index Terms. You can try access to this through your school and check out the terms that may interest you.

False positive results always appear. That's why you should also learn the Boolean operators. They can help you refine your search. For instance, using your attention span example, you can try: "attention span" and "adult" or "attention span" not "educat*" (* is usually a wild card, it's to capture any words starting with educat~, including educate, education, educator(s), educational, etc.)

To look for some starting keywords, I will try Google Scholar and Wikipedia. Once you have some seeds, it should not be a problem to snowball for more.

Also, document your search processes. If you ever want to come back to this search, with all your keywords and database written down it'd be a lot easier. Lastly, since most databases allow us to look inside a particular journal, asking field experts for some recommended journal titles will also help you narrow down the search very quickly.

Remember this is not a one-shot activity. For most of the academics searching for information is an iterative process. So, spend some time to play with the search functions, and don't be afraid to get your hands dirty.

Disclaimer: I am not a psychology expert but since OP uses psychology as examples I just rolled with it. If I wrote anything that's outrageous in this answer please feel free to post a comment, I'll promptly correct them.