Learn Croatian: Kupit ću par jabuka. - I will buy a couple of apples.

The noun jabuka (apple) is a feminine word ending in -a. In Croatian, the genitive plural for most regular nouns is formed by adding -a. For many feminine nouns, this means the genitive plural looks exactly the same as the dictionary singular form in writing (though the final -a is drawn out longer in pronunciation). You know it means "apples" here because of the quantity word par (a couple).

Why is the genitive case used for apples here?

In Croatian, words that express a quantity, like par (a pair / a couple), malo (a little), or numbers from five upwards, require the noun that follows them to be in the genitive case. Because you are buying a couple of apples, you must use the genitive plural form jabuka.

Why is the verb written as kupit ću instead of kupiti ću?

This is the standard way to form the future tense! It uses the infinitive verb kupiti (to buy) and the helper word ću (I will). When the helper word comes immediately after an infinitive that ends in -ti, the final -i of the infinitive is dropped. That is why it is written and spoken as kupit ću.

Do I need to add the word ja to mean "I will buy"?