There are two main ways to search for archival collections: in databases and other general search locations, or directly through specific repositories.
Since it specializes in archival holdings, ArchiveGrid is the best place to start for a broad search across many repositories.
You may also want to think of specific archival repositories that are likely to hold records related to your topic. For example, if you are researching an individual, consider inquiring at the university they attended, or the state or local historical society in the place they're from.
Here is a list of some types of archival repositories and the types of records they usually make available:
You can always start by searching for the same keywords you would use in the library catalog or another search engine. However, because archival collections are frequently organized by creator and not subject, you'll usually have the best luck if you come up with a list of people and organizations involved in your topic. The records and papers of those people and organizations are most likely to contain information relevant to your project.
Example: Women's Suffrage
Searching on ArchiveGrid for "women's suffrage" gives us a large number of fairly general results. Let's think of some people and organizations that might have created archival documents related to women's suffrage:
We have now identified a more limited range of archival collections that are likely to have content specific to our topic of research. As you can see, many archival repositories have holdings related to Elizabeth Cady Stanton. You'll want to look at the size of the collection (often expressed in linear feet) and the summary of its contents to start narrowing down which of these holdings may be of interest to you.
Search tools like ArchiveGrid and repository websites will often lead you to a guide to the archival collection, known as a finding aid. Go to the next page of this guide to learn about using and interpreting finding aids.