i really like Mandarin, one way to tell the difference quickly (to a non Chinese ear) is that Mandarin has lots of soft sounds and sounds a lot like someone whispering, it's really beautiful.  Cantonese sounds completely different to my ear; it has sounds that are half way between Mandarin and Vietnamese (which i also love a lot).

 qetadg:
I just checked out what would be good to start with, so I ended with Autumn´s Concertto. Any there any other good dramas where they speak Mandarin? Can I really assume that every Taiwan and Chinese drama is in Mandarin? Also, does Mandarin and Cantonese really differ that much? What I know is that Mandarin has change in pitch, when in Cantonese the pitch does not matter.

Mandarin and Cantonese are very different. For example, "I" is "wo" in Mandarin and "ngo" in Cantonese. "You" is "ni" in Mandarin and "lei" in Cantonese. (CMIIW).

If you want to hear an example, you can watch this video: https://youtu.be/g9GlfuukikY?t=73
It's a comparison of two versions of "Heavenly Sword and Dragon Saber", the 1986 version in Cantonese and the 2019 version in Mandarin. The subtitle is in Indonesian, but the video clips being compared are in their original languages. Because the clips covers the same scenes, you can hear how different they speak the same sentences.