Learn Italian: Ascolto l'insegnante. - I listen to the teacher.

In Italian, the subject pronoun (like io for I) is usually dropped. You don't need it because the verb ending already tells you who is doing the action. The -o ending on the regular -are verb ascoltare makes it clear that it means I listen.

The English translation says listen to, but where is the word for to in Italian?

The Italian verb ascoltare directly translates to to listen to in English. The to concept is already built into the verb's meaning, so you don't need to add an extra preposition before l'insegnante.

Why does the word for the look like l' in l'insegnante?

Because insegnante starts with a vowel, the singular definite article (the word for the) drops its own ending vowel and uses an apostrophe to blend smoothly into the next word. This happens for both masculine and feminine articles before a vowel.

Based on this sentence, is l'insegnante a male or female teacher?

It could be either! Insegnante is a noun that ends in -e, meaning its spelling doesn't change based on gender. Because the article l' is also used for both masculine and feminine words starting with a vowel, we would need more context to know the teacher's gender here.