I want to create the sharepoint 2010 site on SahrePoint 2010 server. I want to make it available in intranet as well as internet. what exactly i need to do ? is i need to configure mix mode authentication ? how ?
Look at Alternate Access Mappings:
Regarding authentication, you can use Classic Mode Authentication for externally available sites, but all users will need an Active Directory account. Otherwise, you might want to use a web application with Claims Based Authentication that uses a mix of Windows Authentication and Forms Based Authentication.
Note that externally available sites that require authentication are technically extranet sites. An internet site usually means that anonymous access is allowed (at least in part). In the case of SharePoint internet sites, special licensing might be required.