Pushups/situps are pretty useless to be honest, unless you're doing it with weights. Situps are really inefficient, you're using a lot of your back instead of focusing on your abs. That's why crunches are so much better, they focus on your abs doing the work. Getting a six-pack is all in your diet/bodyfat though... situps aren't going to make your abs more visible. Doing 100+ situps is rather pointless, all it's doing is conditioning your body; like weak cardio. Pushups without weights isn't really going to get you anywhere.
Once you start going to the gym, regarding the split, most people have a chest/triceps day, back/biceps day, and legs/shoulders day, because the three major areas of muscles are chest/back/legs, while triceps/biceps/shoulders are minor muscles. Try to avoid doing an "arms" day, devoting a whole day to two minor muscle groups is bad.
With your diet, you must must eliminate sodas, period. So much useless sugar, especially if you drink a lot of it. And don't "slowly" break away from it, just eliminate it. If you just slowly try to get rid of it, you're never going to quit. Unless you drink it in the morning to wake you up or something, it's not like you're going to have major negative side effects like coffee or smoking. Cutting out the junkfood and soda is huge... but you can still snack on shit throughout the day, like someone already mentioned, make PB&J, but MAKE SURE that you get the NATURAL peanut butter, the normal Skippy and shit is loaded with sugar and other bad stuff. Get natural peanut butter and jelly have a couple PB&Js when you're hungry. At school, I usually have that, chicken breast strips (Foster Farms ones are so good, it's just a little pack you can pick up at the grocery store), a gainer shake, and a granola bar or something in between breakfast and lunch.
Make sure not to do cardio immediately after lifting, too.
I'd recommend going to some weightlifting forums and just read and learn... they're a goldmine for info.
Oh, and like someone said, just always keep a waterbottle on you and sip at it all day. Don't chug it, just sip it. If you feel thirsty, you're already something like 20% dehydrated. What I do is just bring a gallon jug to school and keep it in my locker, and fill up my waterbottle in between classes.