I have read about something that selling the computing power but I don't think it gives its value. Are there any more simple methods??
If it is just for playing around, you might look for bitcoin faucets.
If it is for more, then you might look at "https://www.udemy.com/beginners-blueprint-for-earning-bitcoins-online/".
If it is for getting started, and you intend to contribute, look at the code, look at "bitcoin.org", and provide a service to the community :-)
As others have said, you always have to invest something, even if it is just your time, but there certainly are ways to earn bitcoin without any monetary investment.
Here are the 3 sites I use to get all the info I need for this exact question...
A lot of if is the same/similar on each one, but it's definitely some great ideas for earning bitcoins.