How do I set the default button on a web page

[John Svercek]

On an aspx page, I notice when the page opens there is no default command
button (press enter & nothing happens). However, once I set the focus on a
text field one of the command buttons becomes the default (press enter & the
command button is executed). How do I define which button should be the
default and can I change it depending on the text box I am in?

I am using VB .Net for development of a web service that presents the pages.