Firstly good luck with the try outs, Have played goalkeeper before? What to wear for winter football, if its not too cold wear shorts but there's nothing wrong with tracksuit bottoms or even goalkeeping trousers. wear an underarmour under your jersey to keep you warm,gloves are vital as well. Goalkeepers do not need to run any distance maybe 20-25 yards at most,so sprinting is needed. Getting to know your position within the goal is also needed, so say an attack is coming down your left you shouldn't be standing in the middle of the goal. How you save shots and how quickly you can react to things would be another thing I'd be looking at, your communication could be a factor and within your defensive area is vital. Your kicking and throwing I'd look at as well. So quite a few thing to consider which is why being the goalie is such a demanding position to play.
Well, besides from the big things like your diving and reflexes, from what I've seen at camps, tryouts, and practices is that they also admire communication, smart plays, and getting involved in the play. For communication, they want you to assist your players. For example "Back to keep!", "You got him on your left!", etc. For making smart plays, when you get a goal kick or catch the ball, don't just boot it randomly out as far as you can. Be patient wait for your players to get in their positions and pass to the defenders if they are open and have lots of space. To get involved into the play you want the defenders to pass the ball to you if they are stuck and pass it to someone else or clear it if under lots of pressure.
Also for drills have them just take shots at you and throw the ball at you. An interesting drill I saw professional goalkeepers use is have two other players with two balls and a wall. Face the wall about five feet away and have your friends throw the ball at the wall and it will ricochet to you.. It will improve your reflexes and reaction time.
You can wear just about anything, any shirt that is breathable, it can be long sleeved or short sleeved. You can wear shorts or soccer pants, I recommend shorts if its hot. You're going to need long socks and shin guards also. And most importantly your gloves!
Everyone runs, whether it is warm ups or conditioning. You have to be able to run long distances and sprint. Most likely you are going to be in scrimmages and also there might be a special session for only goalkeepers.
I hope this helps and good luck! My tryouts are also soon.