If you learn and practice and master the flying skills in a flight simulator, you will become a certified pilot only for that simulator, not for real life.
You didn't mention the country, but you can look at this answer to find all the requirements to become a private pilot in USA.
Using a flight simulator, you can learn many very valuable skills which will be very useful in your actual flying (and many many pilots do a lot of practice in a simulator), but in no way it can completely replace actual flying. There are many things you can only feel in actual flying experience. You can make any mistake in a simulator and no harm is done, but not in actual flying.
Everything in flying is expensive. Even after you get your PPL, you still have to pay for flying afterwards. Since you are young enough not to have chosen a career path yet (probably), why don't you try to become an airline pilot?
For the answer to your second question, a few topics are:
- the airplane and its physics
- its engine
- effects of weather and altitude on pilots
- ... and many other topics
There are tons of material available covering these topics. FAA has a wide array of reading material which you can download for free.