I want to participate in Open Source projects, but I cannot code (that well. Or.. at all).
I would love to contribute to GitHub projects, but as a designer I'm not sure what's the best way to do this (I mention GitHub because it makes it really easy to find stuff which is already happening, but any platform would apply).
What's the place of a graphic designer in the Open Source world, or what are some good practises when deciding to join a project of these characteristics?
The most important thing is to find a project who's goals you really support. I'd hate for open source work to become a chore, you should quit if it does, and you're more likely to create good work for something you're passionate about. Hopefully the project itself won't die out quickly, because you want to make something that lasts.
Once that is established, you should reach out to the creator(s) and talk to them. Since it's open source, their information is likely clearly visible. I'm sure they'd love for you to help out however you can, and by talking to them and asking, they can tell you exactly how you could help out best.
As for what that may look like, they'd likely be having you do something along the lines of the following (assuming they have work that a designer can do - it's not just code, but includes some type of interface, at least a splash page for people looking at the project):
In addition, there are many design-specific open source projects that help developers. These can range from image libraries (both photography and illustration libraries), to fonts, to templates, to grid systems, to something else entirely. The purpose of these are to make developer's jobs easier and to offer resources so that companies don't have to do everything from scratch by themselves.
What you end up doing, of course, depends on how involved you want to be, the skill set you have, and the type of application you're working on, but I think this answer gives a good idea of what some possibilities of work for you are. It really is the same type of design work you'd be doing for work, just (perhaps) for a better cause.
As Scribblemacher commented, using open source programs and being active in their community by posting tutorials, sample works, templates, questions, new scripts, and feature requests regarding the program helps the application get better. This is something that you could be doing in your regular workflow and is an integral feature of any open source project.