First , don't believe any service that promiss doubling your coins or even those who give you ROI (return of investment) after X days , because they could start legit but they all end up as a scam .
If you want to earn bitcoins with the minimum efforts and you don't have any skills (like programming/designing etc....) then you could enroll into a signature campaign that pays well. (you will need a higher account rank though , a Senior Member or above would be good)