As far as I know, the Goethe Institut has a very good online course for beginners that they don't charge you for. Otherwise I totally agree with Anna, taking lessons is best at the beginning, for various reasons. Firstly, it seems a bit silly to be puzzling about rules with the help of books or grammar sites when you can have a good teacher explain the thing to you in a sentence or two AND practice it with you to make sure that you've understood everything. Secondly, if you have lessons scheduled, you need a lot less self-discipline ;)
However, in my experience, online skype lessons are just as useful as RL classes - if not more so, because they are 1:1.
All the best,
Susanne