Primary sources are the items that get you as close as possible to the answer to your historical question. If you ask "Why did the chicken cross the road?" then the best, primary source might be to ask the chicken (oral interview). This is, however, history and the chicken might have already passed on to the great chicken coop or the chicken might be have an unreliable memory. In those cases, the best source might be the chicken's diary or a newspaper covering that historic road-crossing. In that situation, the newspaper is the document/object that gets you closest to an accurate answer, making it the primary source.
Examples of sources that might be primary include:
Before going to the archive, have the following information prepared:
A good scholar organizes and tags their material as they go. It's a terrible feeling to remember that you read that interesting thing once and to have no idea when or where.
There are two main purposes to your keywords:
Begin collecting keywords before you visit the archive to use as search terms to look through finding aids or archival catalogs. To do this, write out your research question (i.e., why did the chicken cross the road in 1975 in Brighton, MA?).
All those keywords you selected for going into the archive should be used as metadata for sorting and organizing your primary sources. There are a few vital pieces of information you should note and track as you work:
Consider using Zotero and/or Tropy to keep that information in a centralized spot.