You don’t say what language you’re using, but the VB code is below. Use the LoggedIn event, which fires just after the user has been authenticated:
Protected Sub Login1_LoggedIn(ByVal sender As Object, ByVal e As System.EventArgs) Handles Login1.LoggedIn
' Hanging on to the username...
Dim strUserName As String = Login1.UserName
' Getting the user ID (GUID)...
Dim thisUser As MembershipUser = Membership.GetUser(strUserName)
Dim thisUserID As Guid = thisUser.ProviderUserKey
End Sub