It depends what you're looking for. If you don't know, read https://wiki.osdev.org/What_order_should_I_make_things_in https://wiki.osdev.org/What_order_should_I

It depends what you're looking for. If you don't know, read https://wiki.osdev.org/What_order_should_I_make_things_in https://wiki.osdev.org/What_order_should_I_make_things_in and find what sounds interesting to you. Then either start or join a project to do said stuff you find interesting. If you're looking for publications, most academical operating system projects have a bunch of papers (https://wiki.minix3.org/doku.php?id=publications https://wiki.minix3.org/doku.php?id=publications, http://www.helenos.org/wiki/Documentation http://www.helenos.org/wiki/Documentation). Personally I'm having fun hacking on SerenityOS. So far I've done a NE2000 network card driver, net booting, fixes to boot on an old Athlon XP computer, fixes to the ext2 filesystem, printing lines numbers in the JavaScript interpreter backtraces, some keyboard accessibility stuff (https://github.com/SerenityOS/serenity/commits?author=boricj https://github.com/SerenityOS/serenity/commits?author=boricj)... Basically any random thing that I find fun to work on.