これ　それ　あれ　どれ

When you want to talk about things you do not know the name of, you can use such expressions as kore, sore, are, and dore.  Kore refers to something close to the speaker; sore refers to something close to the person you're talking to; are refers to an object that is neither close to the speaker or the listener.

あれはわたしのペンです (That is my pen over there.)
are wa watashi no pen desu.

これはいくらですか？(How much is this?)
kore wa ikura desu ka.

それはさんぜんえんです。 (That is 3,000 yen.)
sore wa sanzen en desu.

Dore means can be used to express "which." Question words like dore and nani cannot be followed by the particle wa. Instead, they must be followed by the particle ga.

どれがあなたのペンですか。(Which one is your pen?)
dore ga anata no pen desu ka.