Filing for the year 2016,what year should be on my W2?

The current W-2 should say "2016".  Did your employer use a batch of old forms?
Is it the standard IRS designed form, or is it a custom form provided by your employer?

Or did you mix up this year's W-2 with last year's?

If it's something you recently received this month, you need to ask your employer, payroll dept., etc. what is going on.   If they issued your 2016 W-2 as a 2015 W-2, they will need to correct that.

2016 information should be on your 2016 W-2. Don't worry if you haven't received it yet - you don't have to file until April 17th (2017) or later, depending on circumstances. 

If many weeks pass and no W-2 arrives, give your employer a call. They're going to be the one issuing the form to you, so they'd know the whereabouts. Ask for HR/Accounting.