Is it better to create your own framework, or would it be best to just use Django or something like that?
You should absolutely use an existing framework such as Django rather than writing your own.
Doing so will let you work faster, and take advantage of code written by others. This will dramatically increase your chances of building a successful website as opposed to spending months tinkering with your own framework rather than solving the unique problems that make your website interesting.
If your site is a big success, you’ll be able to scale Django to meet your demands. There are plenty of huge sites running on Django—such as Pinterest, Instagram, Eventbrite and Disqus. They all end up having their own customisations for scale but that doesn’t mean that they didn’t benefit enormously from using an existing framework rather thank rolling their own from scratch.
Concentrate on getting to a point where your site is so successful that it causes problems. Those are good problems to have!