Here's a small snippet of code to upload a file from the filesystem into a SharePoint document library:
Stream fStream = File.OpenRead("C:\\Temp\\Filename.pdf");
byte[] contents = new byte[fStream.Length];
fStream.Read(contents, 0, (int)fStream.Length);
fStream.Close();
SPList list = web.Lists[DocumentLibraryListName];
Hashtable properties = new Hashtable();
properties.Add("Title", "This is the title");
properties.Add("Other_x0020_Properties", "Some other property");
SPFile destFile = list.RootFolder.Files.Add("Filename.pdf", contents, properties, true);
if (destFile == null)
{
lblError.Text = "<p>Error in adding file. Waiting to re-try.</p>";
}
There is a free class available named DocLibHelper (search Google to download it) that would help you in uploading files among other things, but if all you need is to upload a document into a SharePoint library, then the above code will do.