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

Note: In Japanese, cat = neko, dog = inu. 

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 "ka" to the end. 

Neko desu ka.           Is it a cat?
Inu desu ka.          Is it a dog?
John-san desu ka.    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. 

Neko dewa arimasen.     
Neko janai desu.             
Inu dewa arimasen.    
Inu janai desu.            
John-san janai desu.       

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: 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 (hai) or no (iie).
 

What is it?
It's a dog.

What is it?
It's a cat.

Lesson 1 Vocabulary:

-san

... desu.
... janai desu.
... dewa arimasen.

hai
iie
nan
ka

neko
inu

polite ending for other people's names

It's a ...
It's not a ...
It's not a ...

yes
no
what
particle added to the end of a sentence to make it a question

cat
dog