Hi,
I want to make editable header and footer. I sholud have new tab in top silvewrstripe menu, and in drop down menu options - footer and header, after clicking on them silverstripe should load html editor with my editable footer and header. This options are global, and not connected with any instance of any page.

Is it possible to do something like this? Where should I save the data, and where program the forms to edit this footer and header?

not sure if you want to go about it in this way, but i have done this numerous times.

create a hidden document at the root of your site as a container (this is not necessary, but organizationally nice) called "site resources", then you can create child pages (hidden) under there and use these pages to contain your header and footer data (either use the standard content field or custom fields for other data types).

+ site resources
++ header
++ footer
++ something else

you can then call these pages by name in your template and pull in the content from this page as your header.