If you use iCloud to backup your iPad and store documents, photos, videos, etc., you probably need more than the 5 free GB that Apple gives you automatically. If you use the recently launched iCloud Photo Library, you will almost certainly need more storage.
To purchase more iCloud storage, go to Settings -> iCloud and tap Storage. This will show you your total storage capacity and what you have available. Tap on Buy More Storage at the bottom to upgrade your plan. Available plans are 20 GB for $.99 a month, 200 GB for $3.99/month, 500 GB for $9.99/month and 1 TB for $19.99/month. Tap on the plan you want, then you will have to enter your Apple ID login to complete the transaction.