How can I install Linux on a computer without operating system?

You can use Unetbootin to put the iso of Ubuntu on a usb flash drive and make it bootable. Than once that is done, go into your BIOS and set your machine to boot to usb as the first choice. On most laptops to get into the BIOS you just have to press the F2 key a few times while the pc is booting up.

How do I wipe my laptop and install Ubuntu?

Yes, and for that you’ll need to make an Ubuntu installation CD/USB (also known as Live CD/USB), and boot from it. When the desktop loads, click the Install button, and follow along, then, at stage 4 (see the guide), select “Erase disk and install Ubuntu”. That should take care of wiping the disk out completely.