Also a great place to start is by looking at the code for existing addons. It would be more beneficial to look at 'smaller' addons so you can better see what's happening. In other words, don't open Cartographer first and expect to understand what's going on.
As long as you don't add files, you can modify the addons while WoW is running, and do '/reload' to make the changes take effect. Play around with an existing addon to see what happens.
If you have Auctioneer or Gatherer, they come with !Swatter which is really nice for finding errors. Type /swatter if you have it. If not, look into the BugGrabber or ImprovedErrorFrame addons.