How do you upload a file to a document library in sharepoint?

You can upload documents to SharePoint libraries using the Object Model or SharePoint Webservices.

Upload using Object Model:

String fileToUpload = @"C:\YourFile.txt";
String sharePointSite = "http://yoursite.com/sites/Research/";
String documentLibraryName = "Shared Documents";

using (SPSite oSite = new SPSite(sharePointSite))
{
    using (SPWeb oWeb = oSite.OpenWeb())
    {
        if (!System.IO.File.Exists(fileToUpload))
            throw new FileNotFoundException("File not found.", fileToUpload);                    

        SPFolder myLibrary = oWeb.Folders[documentLibraryName];

        // Prepare to upload
        Boolean replaceExistingFiles = true;
        String fileName = System.IO.Path.GetFileName(fileToUpload);
        FileStream fileStream = File.OpenRead(fileToUpload);

        // Upload document
        SPFile spfile = myLibrary.Files.Add(fileName, fileStream, replaceExistingFiles);

        // Commit 
        myLibrary.Update();
    }
}

As an alternative to the webservices, you can use the put document call from the FrontPage RPC API. This has the additional benefit of enabling you to provide meta-data (columns) in the same request as the file data. The obvious drawback is that the protocol is a bit more obscure (compared to the very well documented webservices).

For a reference application that explains the use of Frontpage RPC, see the SharePad project on CodePlex.

try
{
    //Variablen für die Verarbeitung
    string source_file = @"C:\temp\offer.pdf";
    string web_url = "https://stackoverflow.sharepoint.com";
    string library_name = "Documents";
    string admin_name = "admin@stackoverflow.com";
    string admin_password = "Password";

    //Verbindung mit den Login-Daten herstellen
    var sercured_password = new SecureString();
    foreach (var c in admin_password) sercured_password.AppendChar(c);
    SharePointOnlineCredentials credent = new 
    SharePointOnlineCredentials(admin_name, sercured_password);

    //Context mit Credentials erstellen
    ClientContext context = new ClientContext(web_url);
    context.Credentials = credent;

    //Bibliothek festlegen
    var library = context.Web.Lists.GetByTitle(library_name);

    //Ausgewählte Datei laden
    FileStream fs = System.IO.File.OpenRead(source_file);

    //Dateinamen aus Pfad ermitteln
    string source_filename = Path.GetFileName(source_file);

    //Datei ins SharePoint-Verzeichnis hochladen
    FileCreationInformation fci = new FileCreationInformation();
    fci.Overwrite = true;
    fci.ContentStream = fs;
    fci.Url = source_filename;
    var file_upload = library.RootFolder.Files.Add(fci);

    //Ausführen
    context.Load(file_upload);
    context.ExecuteQuery();
    
    //Datenübertragen schließen
    fs.Close();
}
catch (Exception ex)
{
    MessageBox.Show(ex.Message, "Fehler");
    throw;
}

I used this article to allow to c# to access to a sharepoint site.

http://www.thesharepointguide.com/access-office-365-using-a-console-application/

Basically you create a ClientId and ClientSecret keys to access to the site with c#

Hope this can help you!