Need help about how to learn a new language?

I am 22 years old and know English, Hindi, Punjabi and Bangla. I want to learn a foreign language on my own. Courses etc are not an option for me right now because of the time factor. So the options are either to do it online or invest in a software like Rosetta Stone. As I am keen on this, I don’t mind spending for Rosetta. So is Rosetta the way to go or is there any other choice?

By the way, I chose French :)