Short answer, but it really only scratches the surface:
- The same process that is called "mining" also creates those Bitcoins. Meaning, you actually create them yourself but it can only done by solving a very difficult mathematical problem. The first one to solve that problem will get the reward. It is also important to mention that by solving of that mathematical problem the integrity of the Bitcon network is created (in terms of secure transactions).
- There some exchanges, like Mtgox, BTC-e, Kraken, to name only a few where you can exchange Bitcoin for real money though the process of actually getting the money to your banc account can be very lenghty. There are also also a few Bitcoin ATMs, for instance in Canada of Finland and there are also sites where you can find people in your area who are willing to exchange your Bitcoins for real money.
- This is hardcoded in the software used by the computers which creates the Bitcoin network. Roughly every 10 minutes, a new "transaction block" is created and as fixed reward is issued for this task. Currently this reward is 25 BTC but it will get lower continuously until around the year 2140 it will drop to practically zero. At that point 21 million Bitocins will have been created and that is the total sum of all Bitcoins that will ever be created (unless the software used by the Bitcoin nodes is changed in that aspect).
The most important point to realize about Bitcoin and what many people get wrong in the beginning is that there actually is not thing like a "Bitcoin". You do not have some kind of encrypted files in your wallet that represent a Bitcoin. The Bitcoin network essentially is a kind of distributed ledger system that keeps track of all transactions made within that system resulting on certain amounts of "Bitcoin" associated to a certain address (or account number). If you send somebody Bitcoin, you will actually transfer it from one address to another. Do be able to do that, you need to be in the posession of the private key for that address. This private key is what is stored in your wallet.
But you should really check out some of the great resources about that topic. For instance, check these out: