How to make an impressive Frotoflio file for a startup company?

First, you should probably spell portfolio correctly.
Next? Use your imagination! There are all kinds of websites out there that will give you tips and hints… but it is ultimately your design. How do you want to portray yourself?

There’s probably nothing that I could tell you differently than what’s on those links.

@NaturalMineralWater is right, read your way through the plethora of information that is just waiting for you out there. Maybe once you have done that and have a bit more of an idea, you could come back and ask our opinion on individual aspects?