Learn Italian: Il mio documento e il tuo libro sono lì. - My document and your book are there.

In Italian, you almost always use a definite article (like il, la, i, or le) before a possessive adjective. While in English you just say "my document", in Italian you literally say "the my document" (il mio documento).

How do I know whether to use il mio or la mia?

Possessive adjectives must match the gender and number of the thing being owned, not the owner. Since documento and libro are both masculine singular words (they end in -o), they use the masculine singular forms il mio and il tuo.

Why is the verb sono used here instead of è?

Even though documento and libro are both singular things on their own, combined with e (and) they make a plural subject ("they"). Therefore, you must use sono (they are) from the verb essere (to be).

Why does lì have an accent mark?

The accent mark tells you to stress that vowel, but it is mainly used here to tell words apart in writing. Lì with an accent means "there", while li without an accent is a different word that means "them".