Learn Italian: Compro gli ingredienti per la ricetta. - I buy the ingredients for the recipe.

Masculine plural words usually take the article i. However, because ingredienti starts with a vowel, it must use gli instead. You will also see gli used before masculine plural words starting with z, gn, or s followed by another consonant.

What is the singular form of ingredienti, and how do I know its gender?

The singular is ingrediente. Nouns that end in -e in the singular change to -i in the plural, regardless of their gender. We can tell it is masculine here because it uses the masculine plural article gli (making the singular l'ingrediente).

Could ricetta mean a store receipt since I am buying things?

No, ricetta means a cooking "recipe" (or a medical "prescription"). A store receipt is uno scontrino. This is a very common false friend for English speakers!

Why isn't the word io (I) used before compro?

In Italian, the verb's ending usually tells you who is doing the action. Because compro ends in -o, it already means "I buy." You only need to add io if you want to emphasize that I am buying them, rather than someone else.