Learn Russian: Журналист внимательно слушает писателя. - The journalist is listening to the writer attentively.

In this sentence, the writer is the direct object receiving the action (being listened to), so the noun must be in the Accusative case. Because a writer is a living person (animate), masculine nouns that end in a soft sign (-ь) change that -ь to -я in the Accusative case.

Since both words are masculine professions, how do we know the journalist is the one listening?

In Russian, the endings do the work instead of word order. Журналист is in its dictionary form (Nominative case), which means it is the subject doing the action. Писателя has the special Accusative ending -я, which marks it as the direct object receiving the action.

Why isn't there a word for 'to' after the verb, like in 'listening to'?

Unlike English, the Russian verb слушать (to listen) takes a direct object without needing a preposition. You essentially say 'The journalist listens the writer.' This is another reason why писателя needs to be in the Accusative case!

Can I put внимательно at the end of the sentence?

You could, but placing it before the verb is the most natural word order in Russian. Adverbs ending in -о (like внимательно, meaning 'attentively' or 'carefully') usually go directly in front of the verb they describe.