Dear alansidman,
Really thanks for your instent reply, I know this way to do it but the problem is I want to do it through VBA, because by using Index property (No Duplicates), it returns a VBA error (Debug) due to this code line "DoCmd.GoToRecord , , acNewRec". That's why I want to do it complete through VBA.
So Please review it and Help me 
(QdEvaluators is Query which retrieve only 'Usernames' from original table)
Code:
Private Sub cmdRegister_Click()
If DLookup("[Username]", "QdEvaluators") = Username Then
MsgBox "Username already exists.", vbCritical, "Username"
userFullName.Value = ""
Username.Value = ""
Password.Value = ""
Password2.Value = ""
UserType.Value = "User"
Else
If Not Password.Value = Password2.Value Or Password.Value = "" Or Password2.Value = "" Then
MsgBox "Password and Verify Password should be matched, and should not blank.", vbCritical, "Password"
userFullName.Value = ""
Username.Value = ""
Password.Value = ""
Password2.Value = ""
UserType.Value = "User"
Else
If userFullName.Value = "" Or Username.Value = "" Then
MsgBox "Name/Username fields should not blank.", vbInformation
Else
MsgBox "Congratulations, New user is successfully registered.", vbInformation
DoCmd.GoToRecord , , acNewRec
DoCmd.Close
End If
End If
End If
End Sub