Good is an adjective, and well is an http://www.grammar-monster.com/lessons/ladv.htm">adverb. Therefore, if you are describing a thing, use good. if you are describing an action, use well.
e.g. That was a good test. / He performed well on the test.
It's like slow (adjective) and slowly (adverb).
That is a slow bus. / This bus moves slowly.
Be aware, however, that well is also an adjective which means healthy. This confuses the issue a tad.
This is a good dog.
This is a well dog. (ie healthy).