How can I delete everything and start from scratch?

You are not too stupid to figure it out. Quicken is a deceptively complicated program. Not by design, but because it takes on a very complex financial industry and tries to make it easier. It takes a rather long time to gain a solid understanding using Quicken. The best suggestion is to have a test file to use as a sandbox to gain understanding and a work file to use after you see the expected results from the sandbox file. It's double work, but after a while, you will rely on this less and less.

To answer your initial question, simply go to File>File New and that will get you a "clean slate". Learn to make frequent backups so that if you do create an issue, you can simply restore back to before the error.

Finally, this community is here to help new users, just for the asking.