When we talk about "credible" sources, we mean those which are accurate and trustworthy, coming from a reputable authority. If you work within the library databases, you don't have to worry about this as much, because information sources have to be selected for inclusion in the database collections. Google, on the other hand, isn't picky, and it doesn't care about accuracy -- which means you have to work a little harder to get good information.
You can streamline the quality of your Google searches by focusing on government (.gov), education (.edu), and organization (.org) domains in your results.
Google has some advanced search commands to make this quicker. Just add site:___ to your search! E.g. site:nasa.gov or even just site:.edu. Try it below!
Generally, for scholarly information, you'll want the databases for sure. You can find some via Google, but it's very likely you'll hit a paywall in the process... and these articles are not cheap. Don't do that. Search these instead.
Wait, are these credible or or are they scholarly?
Books are just a medium rather than a class of information. There are scholarly books, there are credible non-scholarly ("popular") works, there are reference works (e.g. encyclopedias -- which are not usually appropriate for your final works cited), and there's junk books. Nonfiction is not the determiner of scholarly.
Scholarly books, like scholarly articles, will be authored by subject-matter expert (PhD not hobbyist), provide copious citations, and will be published by a university press (e.g. University of Texas Press) or a professional society (e.g. the American Psychological Association).
These databases will generally start you out with a basic one-field search not unlike what you see when you go to Google. Don't fall for it. There are 2 big benefits to database searching: you don't have to scrutinize your results for reliability, and you have the power of the Advanced Search to weed out the irrelevant.
When you're constructing your search, think about how to break your topic down into little bite-sized chunks. You can't just write "Civil War Sherman burns Atlanta" -- well, you could... but it's more effective to break it down, as shown below:
As you add on additional fields for search terms, they'll be joined together by AND by default. This "AND" is the most powerful limiter: the articles you get back have to have term 1 AND term 2 AND term 3 to end up in your results.
If you notice you're getting a lot of articles about Gettysburg instead of Atlanta, you can add another field and switch the AND to a NOT (NOT Gettysburg) to help get rid of those results.
All those terms and phrases and ideas you developed before? They all can feed the Advanced Search machine. Be sure to consider synonyms for your different search terms so you can swap them out to play with your search. (E.g. "college" but also "university" or "higher education")
Plus, because you did that background research, you're starting off with a clearer idea of what information you need to find. This means you'll know what you need to put in those search fields to narrow it down from the massive and generic "Civil War" to "Civil War AND United States AND Sherman AND Atlanta."