. . . How can I earn some coins, on the user's transaction? . . .
- You can sell stuff you own and accept payment in bitcoin.
- You can provide a service that someone else wants and accept payment in bitcoin
- You can exchange your local currency for bitcoin using a bitcoin currency exchange
- You can mine for bitcoin if you have a really good graphics card (or FPGA) and access to really cheap electricity.
- You can beg for bitcoin and hope that some kind soul will donate to you.
My BTC client is up and running since one year, but I can't see any coin, for the moment...
And you never will. The current Bitcoin-Qt client does not mine for bitcoin.