We learned how to say "X is Y" by using "X は Y です" but what if we wanted to say that X isn't Y? It's simple. You simply replace です with じゃありません (jya arimasen)
わたしはがくせいです。 (watashi wa gakusei desu)
I am a student.
わたしはがくせいじゃありません。 (watashi wa gakusei jya arimasen)
I am not a student.
For now, keep in mind that じゃありません is used for nouns, so Y has to be a noun. In the case that Y is an adjective (such as 'X is fast'), we will learn about this later. Remember, there are い-adjectives and な-adjectives and they are different in conjugation. じゃありません is used for な-adjectives, but I don't want to start confusing you so don't worry about it until later.