How to generate Random Number in SQL Server

I have the following code:

 Private Sub autogenerated()
        Dim connStr As String = ConfigurationManager.ConnectionStrings("SY_InventoryConnectionString").ConnectionString
        Dim str As String
        Dim com As SqlCommand
        Dim count As Integer
        Dim con As New SqlConnection(connStr)
        str = "select count(*) from Customer"
        com = New SqlCommand(Str, con)
        con.Open()
        count = Convert.ToInt16(com.ExecuteScalar()) + 1
        txtcustid.Text = "000000" + count.ToString()
        txtcustid.Enabled = False
        con.Close()
    End Sub

I want when i try to insert to database let the code check if custid already exist