Your 2000 LS is an OBD-II car (anything sold in the US after 1996 is OBD-II) and therefore the codes cannot be read out without a scan tool. Most chain auto parts stores will read the codes at no charge.
Depending on your skills, tools, and bravery, you can do the job yourself. You'll want to get the updated valve cover gasket set though which is less prone to leaking.
Details on the proper gasket set as well as how to do the job have been posted many times. Use the "Search this Forum" utility from the LS forum main page to search for "valve cover gasket" and you should get a bunch of hits. Some of those hits will point you to the procedure to replace the gaskets.
-Rod