Typically, something like this:

 

When at the top of page, it could be something like this:

 

And as you scroll down (if the header is sticky) or scroll back up (if semi-sticky), then it can look like this:

 

What I’d recommend is keeping at least row 1 sticky, and row 2 (and optionally row 3) semi-sticky. See best practice #14 for more details about sticky and semi-sticky headers.

 

Amazon’s header on mobile is great inspiration: