Play as many games at local places as you can to get an idea of what you really like.

Focus on your pricepoint and a game you think you will like long-term. Wait for it and pounce.

Check all the usual things (boards, playfield, working condition) and ask questions. Bring along a pinball friend if possible.

Clean with novus 1 and novus 2. I also like simple green (in place of novus 1, occasionally).
There are tons of wax...and you can certainly find many forums on it. It seems that pinside has agreed recently that blitz one grand wax is the best. I also like mothers and maguires, however.

Do you have any idea of what games you might be interested in or what budget you are wanting to work with?