Hi Rafał, 

• as to the menu: on Step 2 - Template > Customization >Page scroll options you can select the Keep always visible option
• as to the header, you cannot unfortunately have a fix header only using the program features, still you can use a custom code to achieve this result. Unfortunately I cannot help you with the use of custom code, since the functions do not depend on WebSite X5, but you can search useful information about the custom code with search engines like google or wait until someone on the community of Help Center can help you.

I wish you a lovely day. Kind regards.