As I learn German myself, I’m making notes of what online resources I find useful, or think are worth a look, and adding them here.
None of the links are affiliate links (which means I don’t get paid for recommending them to others).
But go form your own opinion!
Duolingo.com is very popular. It costs nothing, and the website version is even free of ads.
Some people swear by Memrise. Personally I didn’t get on with it, but it’s certainly worth a look.
If you use a smartphone or tablet to study, there’s often an app version of the websites above: