In Chinese grammar, the verb to be is not used in Chinese the same way as it is in English. In Chinese, 是 is for connecting nouns, and is generally not used with adjectives.

Structure
The structure for connecting nouns with 是 is:

This is equivalent to "Noun 1 is Noun 2" in English.
Chinese does not conjugate verbs. That is, the form of the verb is the same no matter who is doing it. In this case, it is always 是 and never changes. As you can see, it's easy to form simple sentences expressing to be in Chinese.

Examples
Simple 是 sentences

 

Noun 1 是 Noun 2   Translation
我 是 学生 。 I am a student.
她 是 医生 。 She is a doctor.
他 是 老师 。 He is a teacher.
这 是 书 。 This is a book.
那 是 杯子 。 That is a cup.

Be careful though. As you can see above, 是 is only used to link two nouns. It cannot be used to link a noun and an adjective. This is a very common mistake for people just beginning to learn Chinese. For that kind of sentence, you'll want to use the linking word 很.