Set Master page when new sub site is created

Step 1:
Create Master page using following URL
http://msdn.microsoft.com/en-us/library/gg447066.aspx

Step 2:
Create Event receiver(right click -> Add New Item-> Event Receiver)

Step 3:
Choose Web events as Event Receiver type
and select "A Site was provisioned"

Step 4:

    public override void WebProvisioned(SPWebEventProperties properties)

        {

            SPWeb topLevelSite = properties.Web;

            string WebAppRelativePath = topLevelSite.Url;

            if (!WebAppRelativePath.EndsWith("/"))

            {

                WebAppRelativePath += "/";

            }

            topLevelSite.MasterUrl = WebAppRelativePath +

                                          "_catalogs/masterpage/Muthuvel2SpCustomMaster.master";

            topLevelSite.CustomMasterUrl = WebAppRelativePath +

                                   "_catalogs/masterpage/Muthuvel2SpCustomMaster.master";

            topLevelSite.UIVersion = 4;

            topLevelSite.Update();

      }

Step 5:

Deploy the application.

Step 6:

Go the the application and create new site. The master page applied for the newly created site.

REF URL: http://www.sharepointkings.com/2012/04/implement-sharepoint-2010-site-event.html