In order to find a Discord User ID you will first need to enable Developer mode on your Discord client. The setting for Developer mode is found in different tabs within the main "Settings" menu depending on what device is being used.

Once Developer mode is enabled, on PC you can right-click any Discord profile and copy their User ID. If you're on mobile, simply press down on the Discord account you want to copy the ID for, then press the 3 dots and finally press the Copy ID button.

For those on an PC: Developer mode can be enabled in the Settings > Advanced Settings tab

For those on an iOS device: Developer mode can be enabled in the Settings > Appearance tab

For those on an Andriod device: Developer mode can be enabled in the Settings > Behaviour tab