You select your water sources. It's not so hard as you think. The old mines and logging camps are often part of the history of an area, which rangers often know, and rangers will be able to tell you about water quality - there's usually nothing really to worry about in the way of chemicals in the Sierra. The only way I'm aware of to remove chemicals is through distillation, which you aren't going to be able to do without a unit so heavy and expensive it's not worth carrying.

You worry overmuch - food is easy. Take things you already eat, typically things that don't crush easily. Those Orowheat thin bread rounds may as well have been made for hikers. Tortillas or round flatbread from Trader Joe's too - fit in bear cans nicely. Boil in bag meals from Trader Joe's too, if you like 'em.

Worry about food for long trips, which you would do after you get more experience on short ones. I spend less on backpacking food than I do on food at home, actually, thanks to the bulk section at Winco, with the dehydrated hummus, trail mixes, candies, bars, etc. Shelf stable meats are easy to come by, and cheese keeps longer than you think, especially if you buy the Tilla-moos or cheese sticks.

As for gear, it costs as much or as little as you want it to. Most of the expense will be in the sleeping bag and shelter and pack. Some people do dollar store and clearance sale items only, some go high dollar, some find a compromise in between. That's why I gave you Mark Verber's website above - he has gear lists for cheapskates, gear lists for ultralighters, gear lists for winter....

A little advice here - don't think about winter at all. Yet. Focus on late spring and summer, and early fall before the snow starts to pile on. It's not that it's just a different gear list - it's a different skill set, and the margin for error becomes much thinner. When you start to go, make sure it's with people who've done winter backpacking before.

Another place to read - backpackgeartest.org. Reviews of many current gear items done by people who've used them for at least five nights - unlike the reviews you find on products on the REI website.