<authentication mode="forms">
<forms LoginUrl="PgLogin.aspx" DefaultUrl="Any webform" name=".ASPXFORMAUTH" Protection="All" Timeout="is by default 20min,you can set it">
<credential PasswordFormat="MD5">
</credential>
</forms>

//Now set this code onto the Logout button
BtnLogout_Click()
{
FormsAuthentication.Signout();
}