Learn Italian: Il mese scorso ho venduto la macchina.

This is the passato prossimo, the most common past tense in spoken Italian. It works a lot like "I have sold" in English. It uses a helper verb, which here is ho (I have), followed by a past participle. Since vendere (to sell) is a regular verb ending in -ere, its past participle changes the ending to -uto, giving us venduto.

Why does the sentence start with Il if the English translation is just "Last month"?

In Italian, time expressions using words for "last" or "next" require a definite article (the word for "the"). You literally have to say "the month last", which is why you need the masculine singular article il before the masculine noun mese (month).

Why does scorso come after mese?

Scorso means "last" or "past" and is an adjective describing the month. In Italian, most adjectives are placed right after the noun they describe, instead of before it like in English. Notice that it also ends in -o to match the masculine singular noun mese.

Where is the word for "I" in this sentence?

You do not need to say io (I) because the helper verb ho already tells us exactly who did the selling. In Italian, it is completely normal to drop the subject pronoun when the verb form makes it obvious who is doing the action.