Learn Russian: Вам нужно купить килограмм сахару и немного чесноку. - You need to buy a kilogram of sugar and a little garlic.

These words are using the partitive genitive (sometimes called the second genitive). For certain masculine mass nouns—uncountable things like food, liquids, or materials—the ending -у or -ю is used instead of -а or -я when you are talking about a specific quantity or a part of the whole. That is why they appear here after quantity words like килограмм (a kilogram) and немного (a little).

Is it wrong if I just say килограмм сахара and немного чеснока instead?

No, it is not wrong at all! In modern Russian, the regular genitive endings (сахара, чеснока) are completely correct and very common in everyday speech. Using the -у / -ю partitive ending just gives your Russian a slightly more traditional or conversational flavor, but both are grammatically acceptable.

Why does the sentence start with Вам instead of Вы?

The word нужно means "needed" or "necessary," and it requires the person who needs the thing to be in the Dative case. So, instead of saying "You need to buy..." like in English, the Russian structure literally translates to "To you it is necessary to buy..." That is why вы changes to the Dative form вам.

Why use the perfective verb купить here instead of the imperfective покупать?