You can use the System.IO.Ports.SerialPort class included in .net 2.0 framework to communicate with your RFID. The first thing you should do is go to MSDN library and read the documentation for this class.
Once you have the communication between the rfid and your app done, now you're looking at upload the data to a database... You can use ADO.Net for this... There are plenty of tutorials on ADO.Net floating around on the Internet, read some of them and try to do it yourself... if you still have problems, you can post a specific question here...