Learn Croatian: Tvoja sestra treba bilježnicu, hoćeš li joj ju donijeti? - Your sister needs the notebook, will you bring it to her?

Croatian has a strict order for clitics (short words that cannot stand alone). The question word li comes first. Then, pronoun rules state that the dative case (indirect object, joj for 'to her') always comes before the accusative case (direct object, ju for 'it').

What exactly do joj and ju mean in this sentence?

joj means 'to her' and is the dative case pronoun referring to your sister. ju means 'it' and is the accusative case pronoun replacing the feminine noun bilježnica (notebook).

I learned that je means 'her' or 'it'. Why does this sentence use ju?

Both je and ju are accusative short pronouns for feminine nouns. Croatian often uses ju instead of je when it is next to another word starting with 'j', like joj, simply because it sounds better and avoids a cluster of similar syllables.

Why does the word for notebook end in -u here (bilježnicu)?

bilježnica is the dictionary form (nominative case). Because the notebook is the direct object receiving the action of the verb treba (needs), it changes to the accusative case, which takes the -u ending for feminine nouns.