German is one of those languages where, unlike English, the details matter. The majority of those details will involve knowing the genders of nouns, without which you cannot say much, if anything at all.
Before delving into the murky world of cases, pronouns and declension, we would recommend cementing your knowledge of genders by learning some of the most common German words. You can do this either by topic in the German Vocabulary section, or in general in by playing the Gender Trainer game. This will provide you with a solid foundation upon which to build your grammar skills.
Once you’re confident that you know a decent amount of German nouns and articles, have a look through the topics on the left-hand side to read through grammar explanations and try the interactive German exercises.