How to set a sticky header
The following steps are available for a 1.9 theme version or above
WP Admin > Appearance > Headers/Footers and edit your header.
Select the fixed position for the header.
- Check the "bottom padding" option and save the settings.
How to avoid overlapping issues
Do not forget to set a background color to the header row. Otherwise, the header content will be looking like overlapping the body content.
Note for the 1.8.7 theme users
The sticky header option is located in WP Admin > Appearance > Customize > Header design and check the "Sticky the menu on scroll".