A piece of code is worth a thousand words, right? 

try
{
    // initialize HttpClient for Twitter REST API
    HttpClient http = new HttpClient("http://twitter.com/statuses/");
    http.TransportSettings.Credentials =
        new NetworkCredential(twitterUsername, twitterPassword);
    HttpResponseMessage resp = null;

    // retrieve Twitter friends timeline
    resp = http.Get("friends_timeline.xml");
    resp.EnsureStatusIsSuccessful();

    // print all friends statuses
    var statuses = resp.Content.ReadAsXElement().Descendants("status");
    foreach (XElement status in statuses)
        Console.WriteLine("{0}: {1}",
            status.Element("user").Element("screen_name").Value,
            status.Element("text").Value);

    // update your Twitter status
    string newStatus = "writing my first HttpClient app";
    HttpUrlEncodedForm form = new HttpUrlEncodedForm();
    form.Add("status", newStatus);
    System.Net.ServicePointManager.Expect100Continue = false;
    resp = http.Post("update.xml", form.CreateHttpContent());
    resp.EnsureStatusIsSuccessful();
}
catch (Exception ex)
{
    Console.WriteLine(ex.Message);
}