You need to have a good idea of what you want to make.

Pick up your ingrediants and make sure that you have made a yeast starter if you plan to have a big brew (either high gravity or lots of volume).

I usually write out a rough sketch of the critical hop addition times and ingrediants for the recipe and a quick proceedure list. Here is an example of a typical Brew Data Sheet, and a blank one for you to print. Note, that you can't fill all the values in until after final fermentation, but this is a GREAT one sheet printout that has all the critical stuff I've found I need to run the brew, as well as review years later as part of my records. Promash also helps keep a great record of all this as well as plan out your brew volumes and hop additions for that perfect taste.

Using PBW or other cleaner on the entire system will ensure that it is spotless and ready to brew. Normally, you don't get too much contaminants in the brew frame system that can hurt the beer, because you will ultimately end up boiling it all anyway, but that doesn't mean that some nasty mold or other growth can't add a musty or bad flavoring to your brew, and some chemicals or residual cleaning mater will off flavor the brew. And it is absolutely essential to have your counter flow chiller and anything else that will touch the wort after boiling perfectly clean and sanitized to avoid infection.

Malts are usually at their freshest if you postpone milling/cracking the grain until just prior to mashing. Milling the grain to a medium crush/crack, helps the mash process of converting starches to sugars by making more of the grain core accessible to the enzymes and heat. Too fine of a crush will end up making a cloudy wort and/or a stuck mash more likely. The crush has been blamed for a LOT of the efficiency variability, and I've seen my own brew efficiencies vary dramatically based on the crush.