Conventions come in all sizes and themes. Phoenix Comicon is about as big a convention that I have attended. Even back in the day with my Star Trek conventions, I have not attended something as large as Phoenix has become. The first year we attended my wife and I managed to get a room the same day as the start of the convention. That doesn’t happen any more.
The first step to surviving a Con, make your arrangements ahead of time. If you need a room, get one sooner rather than later. Figure out what you want to do while there and set your schedule. Now this comes close to becoming too regimented and the serendipity of coming across something unexpected is not to be underestimated. Balance is good to avoid missing the stuff you came for and at the same time not feeling like you are back in school.
Make sure you dress comfortably. If you are coming in costume you will be in it for several hours so make sure it is comfortable and that you can satisfy your bodily functions. Astronaut diapers will only get you so far. Even if you forgo the costume wear comfortable clothes and shoes. You will be on your feat ALOT. Which brings me to my final point. Get used to standing in line. There may, there will be, lines to get into sessions, get autographs or photos, to get food and maybe even get the elevator to your room. Be comfortable being around people and remember your enjoyment is is no less important than the other person’s.