I hate to tell you, but the truth is that more than likely you will have a hard time finding people to critique your code, and certainly to do so for free. Contributors on GitHub are there to contribute to interesting projects, not someone's homework. On Stack Overflow you'll get an occasional bite for small, specific questions, but not for someone to review large portions of code.
If you do want to have someone give you personalized feedback, think about paying for it. Don't overlook that you're asking someone to give up their free time. Consider joining an online course, a code academy, a community college course, or finding a tutor. You might be able to find a non-profit type meetup that is looking to foster broader coding literacy and that will tutor for free, but in general other coders are going to want to spend their free time contributing to a project maintained by other proficient developers.