Menu Items that Change

Hi all, I'm kinda new to the forums
What I'm doing is various exercises to learn some HTML, CSS and some ASP.NET in general. I made an asp:Menu with 2 items, this is inside a MasterPage, not a ContentPage.
Item A links to Page A and Item B links to Page B. The default item, of course, is Item A, because it's the first. What I'd like to do is to change the default item (the one shown) to the one related to the page. If I clicked on Page B, when I load Page B the item selected (that is in the MasterPage) should be Item B, not Item A.
Hope I was clear,
-Sigon