Hey Piratemel, I have done some videos showing how to crochet onto a knitted piece and how to do double crochet. If you want to take a look, click here
and look down in the post under the heading of "Learn to crochet video tutorials" and you see one for double crochet stitch and one for how to crochet a border around a knitted piece.
If you are interested in other crochet techniques, the other videos might be helpful too.
Email if you have questions or post here. Good luck!