This booklet summarizes the basic grammar rules and sentence structure of Danish, thus giving you the information you need to build Danish sentences from scratch.
The information provided in the booklet is also a useful supplement to exercise-based language learning, so I recommend using it in conjunction with one of the interactive language learning websites that are freely available these days.
If you are curious about how Danish works, this booklet is a good place to start, as it gives you a concise explanation of Danish grammar.
For those who will become fluent Danish speakers, I recommend reading this booklet to get a rough idea how the language is constructed, and then spending the majority of your study time with an interactive language learning app.