Pronouns are used to substitute nouns. We use yo for I and tú for you. The most common place we'll use them is before the verb.

Yo estoy casada. (f) - I'm married.

Yo estoy despierto. (m) - I'm awake.

We don't have to say yo and tú before the verb, since the verb tells us who is performing the action. In the examples below, yo and tú are left out without changing the meaning:

Yo quiero comer. - I want to eat.
Quiero comer. - I want to eat.

Tú quieres salir. - You want to leave.
Quieres salir. - You want to leave.

Usually, we leave yo and tú out when no emphasis is required and where no confusion is possible, as:

Necesito un bol. - I need a bowl.
Estoy feliz. - I'm happy.
Estás contento. - You are happy.