Learn Russian: Это кровать. Где она?

In Russian, all nouns have a gender, even objects! The word for bed, кровать, is feminine. Because it is a feminine noun, we use the feminine pronoun она to say "it", just like we would use она for "she" when talking about a woman.

How do I know that кровать is feminine? It ends in a soft sign (-ь), not -а or -я.

Nouns ending in a soft sign (-ь) are tricky because they can be either masculine or feminine. There is no simple rule to tell them apart just by looking at the ending, so you have to memorize the gender when you learn the word. Кровать is one of those words you'll need to remember is feminine.

If кровать is feminine, shouldn't the first sentence be эта кровать instead of это кровать?

Not here! When you are simply stating "This is a [thing]", you always use the neutral, fixed word Это, no matter what gender the noun is. You only use эта when "this" acts like an adjective describing a specific item, like in "This bed is big" (Эта кровать большая).

Where is the word for "is" in these sentences?

In Russian, the present tense of the verb "to be" (am, is, are) is almost always left out. So "This is a bed" is literally just "This bed" (Это кровать), and "Where is it?" is literally "Where she?" (Где она?).