How can I make sure a Username isn't already being used?

I am using SQL Server to store my usernames and passwords.  How do I make sure a username isn't already being used?  Currently, the user gets redirected to an error page I have created.  I would like to keep the user at the current page instead, so s/he doesn't have to re-enter all of the initial information.

I am using VB and am still relatively new.  So, please include as much detail as possible.
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