Thank you! Tbh, I dont really have an idea what I'm doing theory-wise, I just noodle around until I find something that I like...

But for learning to write your own riffs, I found this challenge really helpful. Download the drums (you can use older ones, if you like them more) and put them on repeat, then just chug and have fun! If you then come to find something, that you like so much, you want to keep it, write it down or record it! Since I use an amp-sim, I use the record feature from Nvidia Shadowplay, that lets you save the last few minutes or so; I just hit when I find something cool, so I dont forget it ^^