Connect to database in VB

Long ago, I needed to connect my database in VB and well, the best way is to use app.config and a ConnectionString in it.

In app.config:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
 <startup> 
 <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
 </startup>
 <connectionStrings>
 <add name="ConnectionString" connectionString="Server=myServerAddress;Database=myDataBase;Trusted_Connection=True;" providerName="System.Data.SqlClient"/>
 </connectionStrings>
</configuration>

Protected Sub LinkButton1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles LinkButton1.Click


 Dim myCommand As OleDbCommand
 Dim intUserCount As Integer
 Dim strSQL As String

 strSQL = "SELECT COUNT(*) FROM users " _
 & "WHERE usernameID='" & Replace(txtUsername.Text, "'", "''") & "' " _
 & "AND passwordID='" & Replace(txtPassword.Text, "'", "''") & "';"

 '-> here is the problem
 myConnection.ConnectionString = ConfigurationSettings.AppSettings["ConnectionString"]
 Dim conString As String = ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
 myCommand = New OleDbCommand(strSQL, conString)

conString.Open()
 intUserCount = myCommand.ExecuteScalar()
 myConnection.Close()

 If intUserCount > 0 Then
 lblInvalid.Text = ""
 FormsAuthentication.SetAuthCookie(txtUsername.Text, True)
 Response.Redirect("default.aspx")
 Else
 lblInvalid.Text = "<p class='message'>Sorry... try again...</p>"
 End If
 End Sub