Start off by reading over the preamble to the Constitution. The preamble outlines the reasons why the Founding Fathers wrote the Constitution. In particular, pay close attention to the terminology. Your teacher may ask you to explain "domestic tranquility" and "secure the blessings of liberty to ourselves and our posterity." You will also find it useful to memorize the preamble, if you haven't already. Your teacher may have shown you the "Schoolhouse Rock" song "The Preamble." It's extremely catchy and does the trick.
You probably won't be tested on every single aspect of the Articles of the Constitution, but you will need to, at the very least, concentrate on the main points. These include the structure of government, the system of checks and balances, the roles of each branch and its participants, and the voting process, as well as the extent of federal and state powers. You may also want to think about why this particular system of government was set out, considering the original grievances with King George III and his government.
As you move on, read through the first 10 amendments to the Constitution, otherwise known as the Bill of Rights. Your teacher may ask you which amendment prohibits unreasonable searches and seizures; allows the freedoms of speech, religion and assembly; or grants the right to a fair and speedy trial. In addition to the Bill of Rights, other amendments of note include the 13th, which prohibited slavery; the 15th, which allowed persons of color to vote; and the 19th, which gave the vote to women.
Quizzing yourself is a great help when studying for a test. If your teacher gave you a study guide, use it to make up your own test. Making up flash cards is useful for studying the amendments. Tests and quizzes on the Constitution are also available online.