I checked bitcoin's site a little, and I don't quite understand how do you get bitcoins in the first place... or more important, how the global amount of bitcoin currency evolves.
1. Create (discover) bitcoins using your own computer. This is slow and requires a lot of processing.
2. Provide a good or service in exchange for bitcoins.
3. Receive them as a gift.
That is all. I've ruled out theft, i.e. stealing someone's wallet from their computer.
That answers your first question. The second question, about how the supply evolves, is more complex. My understanding is cursory, but I'm pretty sure the bitcoin.org web site addresses it in depth.