haha.. welcome newbie! im not the expert here but ill say some tips for starters:
eat 6 times a day instead of 3. (smaller meals of course)
you want lots of protein and carbs (if you want to gain) and some fats
eat the right proteins (lean meat, fish, whey)
eat the right carbs (complex-no sugar)- oatmeal, wheat bread, brown rice, whole wheat pasta
eat the right fats- omega 3 and 6 sources (flax oil, fishy type stuff)

talk to the trainer at your gym, get him to walk you through a couple of days and learn what exersizes work what parts of your body. learning about human muscular anatomy will help a lot as well. get into a set routine and start training!

ps. someone should move this to training or diet board?