how to use jquery in asp.net?

You can include the script file directly in your page/master page, etc using:
 

<script type="text/javascript" src="/scripts/jquery.min.js"></script>

 
Us use a Content Delivery network like Google or Microsoft:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4.2/jquery.min.js"></script>

or:

<script src="http://ajax.microsoft.com/ajax/jquery/jquery-1.4.2.js" type="text/javascript"></script>