Follow the following steps
1. Download the Marathi font from here
http://www.marathimati.com/Help/Download-Free-Marathi-Fonts.asp
I have downloaded Roupya Font.
2. Convert the ttf file to eot (for IE 8 or earlier) from the following site
http://www.kirsle.net/wizards/ttf2eot.cgi
3. Copy both Roupya.ttf and Roupya.eot files to a folder named Fonts in your project as shown below

4. Then you need to do as I did on the page
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
@font-face
{
font-family: Marathi;
src: url("Fonts/Roupya.eot") /* IE 8 */;
}
@font-face
{
font-family: Marathi;
src: url("Fonts/Roupya.ttf") /* CSS3 supported browsers */;
}
</style>
<style type="text/css">
input
{
font-family: Marathi, Verdana, Arial, sans-serif;
font-size: medium;
color: black;
}
</style>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:TextBox runat="server" />
</div>
</form>
</body>
</html>
