Firstly you can get the Document Lib Object from SPList Class.
Then use Files.Add method to add a new file.
If you want to add some properties to the file, use Properties.Add method.
Don't forget use the Update method after you change the properties of a SPListItem.
The core code is just like:
SPWeb web = site.OpenWeb();
SPList spl = web.Lists["YourListName"];
spli.Properties.Add("","");
spli.Update();
Pay attention to the difference between SPListItem and File.
Use File.OpenBinary to read the file and File.SaveBinary to write it.
Use File.Delete to delete a file if you want to.
One more thing, if you want a popup window where you can select 'ReadOnly' or 'Edit' when you click the file's link, try to find the DispDocItemExWithOutContext function in JS file.
