First, and most important, this was not a high quality bike when it was new so be very careful about putting a lot of money into it. Unless iyou have a great emotional attachment to it, just clean up and reuse whatever you can and don't consider upgrading very much as it will quickly turn into a money pit.
A new chain, freewheel, tires, cables and brake shoes are about all that can be justified. Thumb shifters or stem shifters can replace the downtube shifters too.
Along with the web sites already mentioned, Bicycling Magazine publishes a good repair manual "Bicycling Magazine's Complete Guide to Bicycle Maintenance and Repair". Some bike shops or Borders or Barnes and Nobel or Amazon book stores should carry it.