Hey @Vinicius ,
Sorry for the late reply; I've been quite busy lately.
Do you have another Macintosh that you can use to create an flash drive installer? If so, then download macOS Sierra on that computer (you don't have to install it, you just need the .app file in the Applications folder), then do the following.
First, insert a flash drive (8GB or above) into the computer. Then, open Terminal (either by using Spotlight, or going to the Applications folder, then the Utilities folder). Once in Terminal, type in "diskutil list". You should see your flash drive in the list. The number you are looking for should look like "/dev/disk4", with 4 being the number of your drive. If you're confused still, send a screenshot of the "diskutil list" output and upload it to the website.
Then, once you get that "/dev/diskX" number, go into Disk Utility and erase the flash drive to Mac OS Extended (Journaled) and with the partition type of GUID Partition Table. Name the drive "USB", as the command you'll run next will be made for the name "USB".
Once that drive is erased, you will now need to run the command "sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --applicationpath /Applications/Install\ macOS\ Sierra.app --volume /Volumes/USB/" and follow the instructions. Once it has finished creating the installer, plug the flash drive into your laptop and boot to it by pressing and holding the Option key (⌥) on your keyboard while it starts up. Select the USB drive, and let it boot up.
When the system is fully booted up to the installer on the flash drive, head to Disk Utility in the Utilities menu at the top of the screen and erase your main drive to Mac OS Extended (Journaled) with a partition type of GUID Partition Table. Quit out of Disk Utility once that's finished, and install macOS Sierra by pressing the "Install/Reinstall macOS Sierra" button.
If there's any issues, feel free to tell me and I'll help you out,