You need to eithe delete apps you no longer use, delete some music, movies/TV shows and/or photos from your iPad.
OR backup your iPad to iTunes on a computer, then decide what you would like to delete off of your iPad.
OR for items like photos, store these to Apple's iCloud remote server storage using iCloud Drive to store any photos or other personal data off site.
Determine how much iCloud Storage space you'll need.
If you have never setup iCloud services, you will need to create an account using your Apple ID and password, then pick a storage plan.
Apple charges for iCloud Storage.
The first 5 GBs of storage is free. The next tier is 50 GBs of Storage for 0.99 per month.
The second tier is 200 GBs for $2.99 per month.
Setting up iCloud on the iPad can be found in the Settings App under iCloud.
Once you have saved the data and have confirmed that it has been stored to iCloud Drive, you can delete those photos and other data off your iPsd to gain back valuable space on your iPad.
Another form of backup for iOS devices is to purchase a wireless WiFi portable hard drives OR Wifi flash memory drives from manufacturers like Seagate, Western Digital (WD) or SanDisk to store your data away from your iPad, but to a local, portable and mobile hard drive.
Good Luck!