What do I need to do with Box 13 if the W-2 has the letter N marked in it?

There should be 3 little boxes in box 13.  N probably means not applicable but ask your employer.   

Box 13 in W-2 is used to indicate whether

• You are a statutory employee
• You have Retirement Plan
• Third-party sick pay

The employer checks the appropriate box to indicate what is applicable to you.