That depends on what kind of learner you are and what you want to achieve. Many people use Teach yourself or Colloquial. I personally don't like those. If you are in Germany you can look into Hueber, Pons, Langenscheidt which are all excellent and offer material like picture grammars or textbooks for self-studying. That's it for the basics.
I would then recommend you grab a copy of your favorite author in German and just start reading. It's best, if you know the story and can use an online dictionary. In addition you should read easy online news like euronews in order to expand your vocabulary. Good luck