Since you do not mention if you're already a pro with OS internals and implementations, I will assume, maybe incorrectly, that you're not.

So in that spirit, try the XinuOS. It has the complete source code for a minimal, clean and practical OS. Search for the book explaining it inside out.

xv6, a re-implementation of Dennis Ritchie's and Ken Thompson's Unix Version 6 (v6) also comes to mind.

And C. Don't forget to sharpen your C skills.