Code for Keeping the Navigation Bar visible while scrolling on website

I am a designer working on a website for a client, currently they are asking for a navigation bar that follows you as your scroll down the pages. Is there any code to make this happen? For reference they are basing this idea off of this website https://www.brassfeedammo.com/.  

Also they would like the product photos to appear smaller and have drop down menus for different types.

Th website password is : Ammo123!

The client does not want this to go live until the website is completely ready.

Thank you!