Primary Sources are:
A first-hand account or testimony of the past, written during that present moment
Something that describes the reactions or thoughts of a particular time period
Primary Sources are vastly different across a variety of disciplines, but the basic definition of what a primary source is stays the same - a testimony or an account describing reactions or thoughts from a particular time. How these testimonies take shape is what changes across disciplines.
At one time, Archives were restricted to serious researchers only. Archives have become increasingly open to any and all researchers. As long as you have a collection in mind to research, most Archives are open to any type of scholar, regardless of your level.
Please be aware that at most institutions, you will need to make a research appointment BEFORE you go. Visit the Archives' website to see how you can visit the collection in person.