Hi,
Questions 1 and 2 can be achieved by editing the template files; if you're using version 1 you can find them in
styles/templates and if you're using the version 2 pre-releases it's
custom/templates.
Template files are pretty much HTML, so you will need to be comfortable with HTML to make modifications. The footer at the bottom of each page can be found in the footer.tpl file, and you can add an icon to the navbar by editing navbar.tpl. Specifically, see the second section of
this post which explains how to add your logo to the navbar.
As for renaming "Donate" to "Store", you can edit
addons/Donate/language.php in version 1, and replace
'donate' => 'Donate', with
'donate' => 'Store',
Hope that helps.