In Japanese, you can turn a sentence into a question by adding the question particle か to the end of the sentence.
Is this a book?
これは本ですか？ kore ha hon desu ka?
The word 何 is pronounced either as nan or as nani in Japanese.
You will get a feeling for when to use which pronunciation soon, but in case you want to know, nan is used before desu and before counters, about which you will learn more later.
What is this?
これは何ですか？ kore ha nan desu ka?
The most common ways to say yes in Japanese are はい and ええ.
はい is more formal than ええ, and can also be used in response to someone calling your name and in response to someone knocking on your door.