iCloud is built into iOS (the iPad's operating system). The first 5Gb of storage is free and should be sufficient for backing up your email, contacts and apps. If you download lots of songs, they will automatically be backed up, but that isn't really necessary, as they can be replaced for free if you lose or change your iDevice.
Photos and your movies are what soak up lots of storage space. You will quickly use up your free 5Gb with those. For 20Gb of iCloud storage Apple charge 79p a month. I have signed up to that as I like you have thousands of photos on my iPad. If you fill 20Gb, the next option in £4.99 a month for 200Gb.
If you have a pc or Mac, you can easily manually back up your photos and movies to your computer instead of going to the cloud, and then delete most of them from the iPad, so it will not keep asking you to buy more storage.
I cannot stress too much how important it is to back up your data from a device. I have known several people reduced to tears when their photos get lost for ever, because they never backed them up.