When you're talking about just having bitcoins then the best source of bitcoin is an exchange with money, real money. When you're talking about having zero starting money, you could start earning by typing of captchas, signature campaigns, trading, gambling. It is a great source of bitcoins
Captchas are boring while signature campaign is acceptable, but thinking that it's a good source of income will only limit your self of exploring greater opportunities here online, we have to be more mature to look for better opportunity that could bring us good returns.
I think captcha can be the alternative if you've completed the work in Siganture campaign or any other job, if you're boring you can listen to music while completing the captcha?