I agree with Lamebeaver on the MRE question. There are also many foods in the grocery store that work very well for backpacking. You'll be surprised what you see if you walk up and down the aisles with backpacking on your mind. Instant dehydrated hummus and refried beans, foil packed tuna and chicken, cous cous, shelf stable meats, etc are all pretty widely available. And it's easier than you might think to make your own. I like the website LB provided, and I also like
Welcome to TrailCooking.com and FreezerBagCooking.com | Trail Cooking.
All that said, I have hiked with people who used MREs. They will work if you really like them, and don't mind the weight/bulk involved.
As for water, the general rule is 2-3L per person, per day. Of course, you do not have to carry all of that at once if you will be hiking near water sources. Just bring some method to treat the water (I realize you probably already know this, but I have met new backpackers who didn't know there were safe/common ways of using water from streams and lakes). If you are bringing food that does not require water for cooking/rehydration, you may be able to get away with a bit less water. OTOH, if the weather is hot, you need considerably more.