it's not a secret actually ,If you browse the forum regularly you can surely find some ways to earn Bitcoin.Your earning amount depends on your skills,If you have some skills like trading, programming, development etc then you will be able to earn more than 0.1 BTC per week.Again, Without investment you can earn up to 0.1 BTC per week by joining a signature campaign.
That is not a realistic number for most users here, since you will probably need to be in the highest paying campaign and your account will need to be legendary to have such a pay and even that may not be possible because it will depend on your commitment to comment in the forum.