Hi there, sjors112,
Thanks for visiting the Apple Support Communities. I understand you're trying to get music on your Apple Watch. I'm happy to help!
You've got a few ways you can listen to music on your Apple Watch. If you want to sync music to your Apple Watch so you can listen to it without having your phone nearby, make sure you're following the steps below from Listen to music, podcasts, and audiobooks on your Apple Watch - Apple Support:
"How to sync music to your Apple Watch
- Set your Apple Watch on its charger and make sure that it is charging. Make sure that your iPhone is within range of your watch.
- On your iPhone, go to Settings > Bluetooth. Make sure that Bluetooth is turned on.
- On your iPhone, open the Apple Watch app, then tap the My Watch tab.
- Scroll to Music and tap it, then tap  Add Music.
- Find the music that you want to sync. Tap the add button  next to the music that you want. If you can't add specific albums or playlists, try downloading them to your iPhone before you sync with your watch."
Make sure that while syncing your music you leave your watch on its charger, and that you keep your iPhone nearby.
If you have your phone nearby, are connected to Wi-Fi, or have a GPS+Cellular Apple Watch, you can also stream music by opening the Music app and choosing your preferred music.
If you still need help, don't hesitate to respond back.
Happy listening!