If you ever get confused when to switch side during the serve, or whose turn to serve, here is a very nice webpage that lists out the rules. It’s only 1 page long with examples.
Badminton online tutorial
There are so many badminton online tutorials out there. Among many that I have read, this site called the Badminton Bible is the most complete and detailed. It contains pictures, detailed descriptions of postures, and also explanations on many things it covers. It is definitely a good starting point to get all your fundamentals down!
There are also many good tutorial videos on youtube of course.