"This is good."
If talking about a feminine noun, such as a cake, would you have to use "זאת טובה"? Or is it also acceptable to say "זה טוב" as a general statement?
As a general statement one would almost always say זה טוב. I would actually translate זאת טובה as "this one is good", to me it means you are speaking about something specific while maybe indicating it, and yes, a cake comes to my mind too. :-)
זה is masculine, and זו is feminine. Feminine nouns end in ה or ת, but there are exceptions. If a noun denotes a woman, it is feminine and if it denotes a man, it is masculine, like mother and father. Names of cities, countries, and languages are feminine no matter how they are spelled. Like in all languages, of course, you're just going to have to memorize the exceptions.