There's a many way to earn Bitcoins. Most successful way is joining campaign. You can start with signature campaign. But you need high quality posts for it. You have to increase your Rank to Sr.Member or above to earn better. After getting some money from campaigns, you can invest it and can earn more. The second way is trading but it requires investment.
I don't think you earn from campaigns without investments. Your time spent on making quality post is an investment and so the only way I think you can earn bitcoins without any form of investment is through faucets where you just have to wait and collect the small satoshis.