Communicate with your partner before after and during the game. Your partner, particularly if an older official, will know what time you should be at the game. However, I like to have at least 30 minutes before the game so I don't feel rushed. Take time to get dressed at the school if possible. Have a pregame, or talk about strange things that you have seen, get your mind on basketball because for the short time that you are on the court nothing else is happening in the world (to you).
For shirts I like
www.theofficialschoice.com, I find that they are the best around. For pants I like Sansabelt. Others may have different opinions. Whatever pants you purchase, get them early because you will need to have them shortened and no cuff at the bottom. Get compression shorts and a black Underarmour like sleeveless shirt for under your stripes. If you prefer to save on shipping there are other shirts out there but I would recommend micro-mesh.