After playing it for a long time and learning everything as said above, Get familair with underneath the playfield because you are going to need to clean that. There is probably a good chance the subway ramp under playfield is dirty. Just remember there's no point cleaning the top if the bottom is filthy.
The one thing I would advise against....seriously advise against, is doing a complete tear down, or taking everything off just because you can. "Oh cool what this? lets just unscrew that and put these parts over here....yippee this is fun!" Seriously. I did that with my TZ which was my first pin, and the damn thing was in tatters for a long long time. It's not the simplest machine to deal with for your first.
I would cliffy the slot and that's aboot it. I don't go for any switch plates personally.
EDIT: Most important for me is taking a TON of pics everytime you take anything apart. In fact, take a ton of pics now so you can have before and after pics. These pics will come in handy at other points to when you forget how a wire was laid.
And make sure to write your sobor self notes when your drunk self starts taking things apart at 3am.