It looks like you've been well advised on clothing etc so i'll skip to the good bit - the actual riding haha.
I've just started again after many many years (25!) out of the saddle - week 5 now and next week, i'm going to start cantering - woohoo!
I went and told my instructor to treat me as a complete beginner as I was under no illusions that I probably couldn't remember anthing, but have picked things up quicker than I thought (or hoped) I would, but go at your own pace and just enjoy. The great feeling when you've had your first lesson is amazing so good luck and don't be nervous (like I was)