1. Wherever you feel it works best - people usually put it in the header or on one of the side menus.
2. Home page text is controlled by the welcome.tpl, while the layout itself is customer/home.tpl (which actually controls the whole site layout)
3. Tabs are controlled by customer/top_menu.tpl and tab.tpl
FYI, webmaster mode will help you find which template is which
