The easiest way will be to create a menu for each top level menu page.
Then put all those menus as blocks in your sidebar region, with a rule so they only appear on the correct page, or subpages.
Top Menu: Home | News | Blog
// show on all pages
Home Menu: Info | About
// show on home and home/*
News Menu: Current | Archive
// show on news and news/*
Blog Menu: My Blogs | Other Blogs | Archive
// show on blog and blog/*
So, step by step instructions would be:
- Create a main menu for your site.
- Add items for Home (URL: /home), News (URL: /news) and Blog (URL: /blog) to your main menu.
- Create another menu called Home.
- Add pages info (URL: /home/info) and about (URL: /home/about) to this menu.
- Add this menu to your theme in a block. Set the block visibility to only appear on "home" and "home/*". This is the clever bit because the submenu for home page will appear on the main /home page, and also any subpages of that as long as you follow the url structure.
- Repeat same process for the news and blog menu so now each of your sub-menus only appear on their specific pages.
- Do a little dance (can even have a sing if you want).