Now that we know how to say what things are, let's try to define what things are NOT. Just like before we start with X　は　Y but instead of using the positive です (it is/am/are), we will use じゃありません (it is not).

So before we said:
すし　は　さかな　です。 --->As for sushi, it is fish.
sushi [wa] sakana desu.

And now we can say:
すし　は　にく　じゃ ありません。 --->As for sushi, it is not meat.
sushi [wa] niku ja arimasen.

As you can see, X and Y are still nouns and the only thing that has changed is the positive です to the negative じゃありません. Let's make a negative statement of our other example.

たけしさん　は　がくせい　です。 --->As for Takeshi, he is a student.
Takeshi-san [wa] gakusei desu.

たけしさん　は　いしゃ　じゃ　ありません。 --->As for Takeshi, he is not a doctor.
Takeshi-san [wa] isha ja arimasen.

What other negative statements can we make? If you already made positive statements from before, try making those negative too!
がんばります！(ganbarimasu!) Do your best!