This simplest structure in Japanese is the "...です。" structure.  です　is pronounced 'dess' as the u becomes silent.  Simply put, ____です　means "It is", "Those are", or "I am." ____ ‚.   It's ___. 

Note: In Japanese, cat = ねこ　dog = いぬ. 

ねこです。
いぬです。
ジョンです。

It's a cat.    or   Those are cats.
It's a dog.   or   Those are dogs.
I am John.

There is usually no difference between plural and singular things in Japanese. To distinguish what is being said, one must rely on context or add an adjective to describe how many dogs or cats or whatevers you have.

To make a sentence a question in Japanese, you add "か" to the end. 

ねこですか。　Is it a cat?
いぬですか。　Is it a dog?
ジョンさんですか。　Are you John?  Is it John?

Note that the suffix 'san' was added to the end of John when it became a question.  This is because you are reffering to someone else, rather than yourself.  It is considered polite to add -san to people's names other than your own.  Be very carefull not to add it to the end of your own name though, as it is considered very rude and unusual.

To say "It's not a cat." or "I'm not John." you have to use the form dewa arimasen  or janai desu.  Either form means the same thing, but dewa arimasen is more formal. Note: The above romanization uses wa in dewa while the phonetic characters below use ha. This is because the phonetic character ha in this case is pronounced as wa.

ねこじゃないです。 
ねこではありません。
いぬじゃないです。
いぬではありません。
ジョンさんじゃないです。

It's not a cat.
It's not a cat.
It's not a dog.
It's not a dog.
I'm not John.

Note that, in the example above, you are not John, therefore refering to him with -san is ok.

If someone asks you "Is it a cat?" you can respond with yes (はい) or no (いいえ).
 

ねこですか。
はい、ねこです。
いいえ、ねこじゃないです。

What is it?
It's a dog.

What is it?
It's a cat.

Lesson 1 Vocabulary:

-さん

... です.

... じゃないです.