want to use Session Variables in Sharepoint?

SharePoint by default does not use session state. You need to explicitly change it in the Web.Config file of your sharepoint web application
<httpmodules>
<clear>
<add type="System.Web.Caching.OutputCacheModule" name="OutputCache">
<add type="System.Web.Security.WindowsAuthenticationModule" name="WindowsAuthentication">
<add type="System.Web.SessionState.SessionStateModule" name="Session">
</httpmodules>

and also

<pages enablesessionstate="true" enableviewstate="true" enableviewstatemac="true" validaterequest="false">