As the title suggests they are a lawyer who defends a person that has been charged with a criminal act.
They have to represent the defendant against the prosecuting attourney who presents the case against them to the jury/judge. Even when there is no defense for the crime the defendant needs a lawyer to represent their best interests in the case.
Any lawyer goes to college takes pre-law classes, the LCAT, and then goes to Law School. After graduating law school they take the State exam to practice law.
If you're interested in this side of the law a good show that represents the ideals of criminal defense is 'Raising the Bar' on the TNT channel.
You should try contacting your local law librarian, if you have a state that has them in the counties (or try the state law library.) Hopefully your instructor has given resources to use.
First, figure out what topic it is you are researching. Then figure out the key words of the topic to search on. Then figure out what courts would be involved - state, federal, etc. Figure out what databases you will search. After you find some cases, you have to shepardize them - in other words find out if any courts have overturned or altered the decisions of the cases you have found, to make sure they are the current law. You also need to look at the cases cited in the cases you find to see if they are closer to your topic or affect your topic. Then you have to shepardize those cases too. Shepardizing is much easier now with computer systems.
You also need to do research on laws that affect the case and if the courts have made any decisions on the laws, following the procedures named above. How you do all of this depends on what databases you have available.