Learn Spanish (Latin America): Yo soy el hermano y tú eres la hermana.

In Spanish, nouns have a gender. Words ending in -o like hermano (brother) are usually masculine, so they use the masculine word for "the", which is el. Words ending in -a like hermana (sister) are usually feminine, so they use the feminine word for "the", which is la.

Do I have to say yo and tú, or can I just say soy el hermano y eres la hermana?

You can leave them out! In Spanish, the verbs soy (I am) and eres (you are) already tell us who is doing the action. Including yo (I) and tú (you) here just adds emphasis and makes the contrast between who is the brother and who is the sister much stronger.

Why does tú have an accent mark over the u?

The accent mark helps distinguish tú (you) from tu (your). Without the accent mark, tu hermana would mean "your sister." With the accent mark, tú eres means "you are."

How do I pronounce the "h" in hermano and hermana?

You don't pronounce it at all! In Spanish, the letter "h" is always silent. You pronounce these words exactly as if they started with the letter "e".