How to reinstall macOS Sierra after formatting by Ubuntu

I recently experienced a problem with my macOS Sierra that forced me to reinstall it. However the internet connection was bad, so I ended up installing Ubuntu 17 from USB instead.

I’ve now reformatted my hard disk and torrented a macOS Sierra installer. How can I burn it onto a USB stick?

I have had a similar situation with a friend of mine who installed a bad copy of Windows 7 onto their mac.

You can retry booting into Internet Recovery Mode. If a local recovery partition isn't found, which in your case it won't be due to the hard drive formatting, the Mac will try to recover the operating system over the internet. To force a Mac into internet recovery mode, hold Option-Cmd-R at startup. This process will take some time, and you will need to select a suitable Wi-Fi or other Internet connection once the computer has started up.

More information on macOS recovery can be found on Apple's support site

Best of luck to you, and note it is probably not the best idea to install a pirated copy of macOS Sierra.

EDIT: As for "burning" things onto a USB stick, this tool may help you.