For online courses, I highly recommend Duolingo (especially if you're a relative beginner) - it's entirely free and without ads as well. I honestly wouldn't even consider paying money for an online course / language learning software before trying Duolingo and seeing if it works for you. Also, as you get more advanced, you can use the Immersion area and practice translating documents which is really good.
As far as online tools go, I would also recommend Memrise and Quizlet for vocab. This little game is good for obscure words as well.
And of course, read the news, watch TV, watch films, listen to music, and read books in the language! I recently finished reading a translation of the first Harry Potter book, fortunately, on my Kindle, as that makes it very easy to look up words in a bilingual dictionary. There are lots of discussions on the German section of the Duolingo forums about good music/films/TV etc. for learners to consume.
If you like your music then lyricstraining.com is amazing. You basically choose a song and a level, and then you see the lyrics going by and have to fill out the missing words (higher levels have more missing words). This is great for training your ear in the language.
Also, I would recommend Deutsche Welle and I sometimes read the news on spiegel.de.
Viel Erfolg :)