When talking about playing an instrument we use jouer de. The de must then agree with the noun that follows:
je joue de la guitare - I play the guitar
il joue du piano - he plays the piano
With food
When talking about food as in buying quantities, de doesn’t require an article to follow it, as here it means of. Normally however, the noun can be in the plural form just as in English. For example:
je voudrais un kilo de pommes - I’d like a kilo of apples
j’ai besoin de 250g de raisins - I need 250g of grapes
je veux un demi-kilo de carottes - I want half a kilo of carrots
Similarly, when reading a recipe, the ingredients preceded by de don’t need an article agreement, as here too de means of. For example:
