You will need to use another mac to make the bootable installer.
Here are the instructions. Read them, read them again and follow them to the letter.
If you do have an operating system on your mac and it is functioning then you can download
Sierra using this method.
How to upgrade to macOS Sierra - Apple Support
Go to Section 4 and click on Download macOS Sierra.
This downloads InstallOS.dmg to your Downloads folder.
The next section can only be done on a mac that is capable of running Sierra,
a mac that came preinstalled with an OS later than Sierra will refuse to do the next bit.
When downloaded open to InstallOS.pkg, double-click on
that and an installation window will open, this does not install Sierra
but converts the InstallOS.pkg to the Install macOS Sierra.app which
you will find in your Applications folder.
(If the installation window asks which disk you want to install to, you must pick
the disk that you are booted to at the time. Not any internal or external disk that
you want to eventually install Sierra on, that is for later.)
Here is the instructions to create a bootable USB installer.
There is a problem with making a bootable USB installer with the Install macOS Sierra.app
where it returns an error Volumes/<USBname> is not a valid mount point. There is a workaround.
With Install macOS Sierra.app in your Applications folder open Terminal.
Paste in this command
sudo plutil -replace CFBundleShortVersionString -string "12.6.03" /Applications/Install\ macOS\ Sierra.app/Contents/Info.plist
press return
enter your password
press return
Now paste in the createinstallmedia command for Sierra where MyVolume can be replaced with the name of your USB stick
If your USB name has more than one word separated by spaces you would need to type it in as
/The\ USB\ Name
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ macOS\ Sierra.app
press return
follow the prompts
make dinner for your dog.