It can't be done on the iPad in the native Photos app. The only way to create folders is to create them on your PC or Mac and then sync your iPad with your PC. Then the folders will be transferred. Similarly you can't delete photos on your iPad that have been synchronised with your PC - they can only be deleted on your PC and then synch'd with your iPad and they will then disappear.
Photos that you received on your iPad that weren't synch'd - say via email - can be deleted on the iPad, but you still can't create folders on the iPad for them.
However there are many third-party apps in the App Store that don't have this restriction; you'll find several reviews and recommendations for them in this Forum if you search.
I know, it's an annoying 'feature' of the Photos app - but, well, that's life I guess!!