Welcome to MobiFAQ! See this guide: [Guide] Recovering Deleted Files
. If you deleted photos from Internal Storage, you have to root the phone in order to use a program like DiskDigger. Rooting is the process of gaining superuser access, which I don't recommend for novices. On top of that, rooting does require writing some data to the phone, which can overwrite any potentially recoverable deleted data.
In the future, make sure you turn on Google Photos Auto Backup, which will automatically save your photos to your Google Photos in the cloud. Remember that phone storage and SD cards are not forever, and can fail or be lost unexpectedly--you should be backing up your important photos and other data on a regular basis somewhere.