how do I delete everything on my disk? i'm pressing delete but everything is still there

Just what do you mean by "delete everything"?

One can't delete everything from your startup disk unless you are booted to Recovery. Then you can erase the startup disk via Disk Utility that is in Recovery and then start over by reinstalling macOS

How to reinstall macOS - Apple Support

once you've saved your data to the external drive, you can then safely delete that same data from your mac's internal drive. just move it to the trash, then empty the trash to free up that space.

You can erase the volume using the Disk Utility (see Erase and reformat a storage device in Disk Utility on Mac).