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Joe, you need to dive in and look around. It's the best way to learn. If you run into a snag, go hit the main forum over at forscan.org and look it up.
There are many, many codes, and you could make a career figuring out what they all do, or you could just peruse the mods that others have discovered (by just paging through all the ones that have been documented, and decide which ones are relevant to you). Another approach is to imagine what you might want to mod, and then go look to see if there is a documented way to do it.
Experimenting with just randomly changing bits is probably not a good way to go.
Whatever you do, before you change anything, download your "as built" vehicle profile first. That way, if something goes wrong, you can set it back to stock and try again.