Hey there!
Let's first say what a primary source really is.
A primary source is a source created by a person who was actually there. They've written about their experiences, and are primary (as in first) because the information is valid. It's proven by a real witness. This is not to be confused with a secondary source-- an account written by someone who wasn't there but has info, like a textbook.
When you're evaluating a primary source, you should ask if the person is writing about their experience in that particular event, and if the information seems valid. You should also make sure nothing is necessarily cited and such- and detail and usually opinion can be present.
Primary source example:
An autobiography- the person whom the story was about was there and is writing about their life.
Hope this helps!