how do you restore from backup provided you have logged in itunes for the first time

Logging into iTunes has nothing to do with restoring a backup. If you mean that you previously synced with iTunes and created a backup when you synced, that's a different altogether.

Restore your device from an iCloud or iTunes backup - Apple Support

1. Connect your device to the computer that you stored your backup on.
2. Open iTunes. If iTunes tells you there's an iOS update, get the latest version.
3. Go to File > Devices > Restore from Backup. If you're using Windows and don’t see a File menu, press the Alt key.
4. Look at the date and size of each backup and pick the most relevant.
5. Click restore and wait for the process to finish. Your device will restart and then sync with your computer. Stay connected until the sync finishes.

I find your phrase "... logged in iTunes for the first time" very confusing.

Are you saying that since you never logged in to iTunes previously, your backup was done to iCloud? If so, the only way to restore an iCloud backup would be to do Settings > General > Reset > Erase All Content and Settings, and then once you set up the iPhone by connecting to WiFi and entering your Apple ID, choose the option to restore an iCloud backup.