Direct Object Pronouns are used to save repeating the names of people/items over and over. Let’s have a look at a conversation:

Me: I bought a book yesterday.
Friend: Where did you buy the book?
Me: I bought the book at the store.
Friend: was the book expensive?
Me: The book was pretty cheap.
Friend: Have you read the book yet?
Me: No I have not read the book yet.

Look at how many times “book” was repeated in the conversation. In English we would use the word “it” instead of repeating “book” over and over. This is the purpose of a direct object pronoun!

Me: I bought a book yesterday.
Friend: Where did you buy IT?
Me: I bought IT at the store.
Friend: Was IT expensive?
Me: IT was pretty cheap.
Friend: Have you read IT yet?
Me: No I have not read IT yet.

Key things to know:
The direct object is the item or person being directly affected by the action of the verb. We can ask the questions “What?” or “Whom?” to determine if something is the direct object.
E.g. What did you buy yesterday? A book. The book is the direct object of the sentence.

We have to have mentioned the item/person before we can replace it/them with a direct object pronoun. We must also check the gender and quantity of the item for the 3rd person singular and plural forms (him/her/Usted/it and them/Ustedes) form to make sure the pronoun reflects these accurately.

Once we have worked this out we can look at which pronoun to use:

Me me Us nos
You (sing. inf) te You (pl. inf) os
Him/her/Usted/it  lo/la Them/Ustedes los/las