The conjunctions that are used to join complete thoughts are red.  The other conjunctions just join words or phrases.

1.  Nancy and Sharon sit at the back of the room and talk, but they don't bother anyone.

2.  Conjunctions combine phrases and clauses, and they also join words.

3.  Sharon and Neil may go for a bike ride, or they may go bird watching.

4.  Dr. Gibbons did not have time to score the assignments, so he cancelled them.

5.  Mrs. Blake just had a baby, so she is tired, but Mr. Blake is tired too. 
(This is a tricky one!  It has three complete thoughts combined with conjunctions.)

6.  I studied the notes for two hours, but I'm afraid I won't pass the test.

7.  "I'm sorry to tell you this, but you have to give up junk food or soda pop," said the doctor.

8.  Mark gave a popcorn-flavored sucker to Jim because Jim forgot his lunch.
(No comma needed here.  The word because is not a conjunction!)

9.  I have a friend who works at the amusement park, and she can get me a free pass.

10. Conjunctions and commas are often used together, but you must know the rules for doing so.

Here are some resources that might help if you are having trouble:

http://englishplus.com/grammar/00000069.htm

http://englishplus.com/grammar/00000323.htm

http://www.chompchomp.com/terms/coordinatingconjunction.htm

Finally, let's talk about other comma rules.