Including unpublished manuscript with CV?
As others have said, it depends on the culture of the discipline you are in. 

I also think it is important where you are in your career. If you are an undergraduate, for example, with a manuscript on which you are an author, I would be impressed even without the acceptance yet. Most undergraduates in my field (biology) have not done much (or any) hands-on research so this would be a big CV booster! Since you say this is your first manuscript, I assume you are early in your career so it may help you stand out from your peers. I have seen graduate students and even professors include submitted manuscripts on CVs and personally have not found it tacky.