@username1001, creating a forum would be an interesting project. That’s something that’s been on my list of things to build, but between all of my other active projects and regular job, I haven’t found the time yet.
I’d suggest to start out small, break up the project into smaller pieces to make it more manageable, create a mockup of the design, and pseudocode some of the logic. The initial design and analysis will help you work through some of the issues you’ll come across because you’ll discover that some things are more involved than they sound when you just have an idea of what you want it to do. Like many other technology projects, this can be an ever evolving project for you. As your skills and knowledge improve, you can add new features or refactor the project to work a different way or implement a technology you learned recently.
It doesn’t matter if there are 1000 other forums out there. This would be a great learning experience that could help you in many other aspects of programming and website design and development. Don’t get discouraged and you can always ask for help when you get stuck on something.
Above all, have fun with it!!