I have successfully  installed DotNetNuke on my machine but when I am browsing my website it is giving following error:

NullReferenceException: Object reference not set to an instance of an object.]
   DotNetNuke.UI.Skins.SkinDefaults..ctor(SkinDefaultType DefaultType) +189
   DotNetNuke.Common.Globals.get_DefaultSkin() +31
   DotNetNuke.Entities.Portals.PortalSettings.GetPortalSettings(Int32 TabId, PortalAliasInfo objPortalAliasInfo) +2797
   DotNetNuke.Entities.Portals.PortalSettings..ctor(Int32 tabId, PortalAliasInfo objPortalAliasInfo) +82
   DotNetNuke.HttpModules.UrlRewriteModule.OnBeginRequest(Object s, EventArgs e) +2882
   System.Web.SyncEventExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +92
   System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +64

Can someone help me with this problem??

That error is caused by a missing configuration section in the file DotNetNuke.config (in your top directory).... find it and add this to the bottom of the file:

<skinningdefaults>
<skininfo folder="/DNN-Blue/" default="Horizontal Menu - Fixed Width.ascx" admindefault="Horizontal Menu - Fixed Width.ascx" />
<containerinfo folder="/DNN-Blue/" default="Image Header - Color Background.ascx" admindefault="Image Header - Color Background.ascx" />
</skinningdefaults>

That needs to be added right above the last line, the one that says:

</configuration>

JK.