I always thought my dog was dog-aggressive, until someone in this forum said that she was possibly just reactive. Question is, how do I train her out of her reactivity? She whines and cries when on walks, and whenever she sees another dog, she will howl, cry and when that other dog gets near, acts aggressively towards it. Having treats with me doesn't seem to work because my dog is too focused/reactive to respond.
Another thing is that although I have trained my dog, she won't follow the "sit" command unless she knows there is a treat that awaits her. I am not overly generous with treats, and I alternate between them and praise, but how do I wean her off expecting treats and get her to follow the command? It's a hit-and-miss thing; sometimes she will sit but sometimes she will not. Does she really understand that "sit" means her butt needs to hit the floor?
Lastly, how do I teach her "wait" before she goes out the door for walks? I've tried the opening/closing the door thing, but she still tries to squeeze her way out when the door is slightly ajar and stands near the door when it's closed. Also, when I put her halter on for walks, she acts like it is a game and tries to avoid it or puts her head up in an effort to see "what is happening" when it goes on. I tell her to sit, but she won't so I just wait for her to calm down until I can 

ut it on. Is this the right thing to do?
Sorry it's quite long, but I'm a first-time dog owner, so you can guess why there are so many questions!