I'm not asking how to make pull requests or anything. Just for advice for someone that wants to introduce himself to the open source world.
So, any tip for picking a first project, without any preferences besides (maybe) the language?
Note: The languages I more or less know are Python, C/C++, Javascript and Go.
Most communities are thrilled to get new contributors and willing to put in the effort to get you pointed in the right direction the first time around. If they aren’t, move on to another project. In fact, skimming the recent mailing list archives can be a good way to get a pulse on the culture of a project and how welcoming it’s community is.
If you don't want to interact with open source project maintainers, you could try writing example programs, tutorials, or guides for products you like - that's another form of contribution.
Contributing to OSS is a bit like working out, when you get into a routine its easy but starting is very difficult. The trick is to just start.
If there's already something on github, join that - you can even do a fork first, and play around.
Otherwise, just build a new project on github, commit to it, and let people know here/reddit - and see if they'll join