How can I factory reset my iphone 8 if I forgot the password?

If you're talking about the passcode to unlock your iPhone's screen:

See these instructions, keeping in mind that if you don't have a backup all data will be lost on that device (except those that you eventually synced to iCloud):

If your device is running iOS 15.2/iPadOS 15.2 or later:--> How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support

Shouldn't be this case (iOS/iPadOS earlier than 15.2), you need a computer. If you don't have a computer, borrow one from your friends/relatives/neighbors or ask for help at an Apple Store or Authorized Service Provider, then read these suggestions.

Given that to remove the existing passcode you must connect your device via USB to a computer (a Windows PC running the latest iTunes version, or a Mac, which can run iTunes or Finder, depending on the macOS installed on it), pay attention to the specific gestures that you need to perform while connecting your device to the computer.

These are the instructions to re-gain access to your iPhone/iPad:

If it is an iPhone:--> If you forgot your iPhone passcode - Apple Support

If it is an iPad:--> If you forgot the passcode on your iPad, or your iPad is disabled - Apple Support

Also, take a look at this support video about the same matter (for both iPhone and iPad):

https://www.youtube.com/watch?v=7s8-zLG9ySQ

If you're talking about a different password, please specify your needs better.

Regards

Giulio

You can't. You need the password to deactivate Find My before you can reset to factory settings.

Activation Lock is a theft deterrent feature with no bypass or workaround.

Without the owner's authorisation (Apple ID & password), no thief or anyone including you can re-use the device.

You can recycle the device responsibly.