If I use a USB stick to install an OS (Ubuntu) do I have to format it first?

Yes, as bootable. All the info and applications you need here. Please read carefully. https://help.ubuntu.com/community/Installation/Fro...

The drive you're installing on, or the flash drive. You don't have to format the flash drive. But, you may want to if you have data on it (either unwanted, or backed-up). With regards to your hard-drive, you WILL WIPE OUT ALL YOUR DATA and OS if you reformat it. Ubuntu's install program will do this for you, or ask if you want a dual-boot system. If you want to keep your current OS, you may want to consider using Virtual Box.

You cannot install Ubuntu on a USB stick, it has to go onto the hard drive. Assuming you have Ubuntu on a DVD, put that into the computer and boot from it. You may need to change the settings in BIOS so the computer knows to look for the DVD gfirst before the hard drive. Once you start Ubuntu and click Install, it will do any formatting for you, you don't need to do it yourself.

When you're putting Ubuntu on the thumb drive formatting is part of the process. Please note that you can't just throw the file on the thumb drive like you would with normal files, you need to use a special program like Unetbootin to make the thumb drive bootable.