What should I do?
My PC is completely filled up with stuff, I have a 2TB hard drive that I have used a TB of already and I think it has had a consequence on my PC.
Should I format it or should I just leave it as it will not make a difference?
What you really need to do is go through your things, find anything that you want to keep, and then format. Deleting a few files here and there can help, but a fresh start with only the things you need will be much better.