Learn Italian: Non ho voglia di questo pesce. - I am not in the mood for this fish.

In Italian, physical states and cravings often use avere (to have) instead of "to be". The idiomatic phrase avere voglia di literally translates to "to have desire of", but it is the standard way to say you crave or are in the mood for something.

Why do we need the word di here?

The expression avere voglia is almost always followed by the preposition di when you are mentioning what you want. It connects your craving to the object. Literally, you are saying "I don't have desire of this fish."

How do I know to use questo instead of questa?

Questo means "this", and because it describes a noun, its ending has to match that noun's gender and number. Because pesce is a masculine singular word, we use the masculine singular form questo.

Pesce ends in -e. How can I tell it's masculine so I can match it with questo?

Italian nouns ending in -o are usually masculine and those ending in -a are usually feminine, but words ending in -e can be either! You simply have to memorize the gender of -e words when you first learn them. Pesce happens to be masculine.