There are many different ways to earn bitcoins, Starting from faucets and signature going through online methods in general.
There are various methods as quoted by above mate. But the best is to initiate your own business with bitcoin acceptance and popularize it.
But starting a business and making it popular are very challenging tasks for the any people even for the experienced one. But the challenging one would be always most rewarding.
We need to start offering services with small scale, from that need to develop as business. This might help us to collect more BTC.