When to refer to a paper as "seminal"
I would understand "seminal" to indicate that  (a) the paper was the the first in some sense, and (b) that it led to a lot of subsequent research. For example, a paper proposes and tests a theoretical idea, and then lots of other people come along and test that idea building on the original study. 

It doesn't matter that you are not established in a field. Anyone can use the word "seminal". It's just that using it correctly requires a good understanding of your field and the interconnections of research papers. 

Having a lot of citations is necessary but not sufficient to show that a paper has led to subsequent research. Some citations don't mean much on its own.