How can I remember all the endings?

It is true that there are a lot of endings for every type of word in Greek. There are many ways to learn them, depending on the person. For example, you could try writing a whole table without looking at the textbook, then checking for mistakes and correcting them. If you prefer a more active exercise, try translating an unseen and analyse the ending of each word. Whatever you do, do not simply stare at the tables and hope to memorise them - learn actively, not passively!