"No, a lemon is not good."
Translation:Nein, eine Zitrone ist nicht gut.
"nicht" negates "gut".
"The most general rules are:
Nicht will precede the specific word that is being negated
If there is no specific part of the sentence that is being negated, nicht will come at the end."
There aren't many rules. But you can find some tables and some general rules (which usually have some exceptions, they aren't "hard" rules) Here's an example: http://german.about.com/library/weekly/aa042098.htm
Why isn't it Nein, ein Zitrone ist nicht gut. I thought Eine was for feminine purposes?