Second the good boots. Waterproof preferably. And a pair of very light sneakers for the moment you actually get into the aircraft. Personally I prefer a pair of zip-the-legs-off outdoor trousers, t-shirt or polo shirt and a windbreaker fleece. No points are awarded for clothing style in the gliding scene!
Bring a backpack with the stuff you need throughout the day (lunch, sunscreen, sunglasses, hat or cap, warm clothing, book, you name it). There is usually no opportunity to get back to the clubhouse to fetch stuff throughout the day unless you fancy a long walk.
A logbook and a pen is necessary but make sure you get the correct one. (The logbook that is - normally any pen will do.) The club can advise. (On the logbook.)
And for the rest, bring patience. What you'll find is that there's one instructor and one plane for each three to five students or so. You'll be spending a lot of time on the ground and during that time are supposed to help hook up the aircraft to be launched, and retrieve the aircraft from the landing field. Do get a proper briefing before you join in those activities though!