We use yo for I and tú for you (familiar). Generally we use it before the verb.

Yo estoy casado. - I am married.
Tú estás feliz. - You are happy. (familiar)

There is not always need to say yo and tú before the verb, since the verb ending tells us who is performing the action.

     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 not necessary, as:

Necesito un bol. - I need a bowl.
Necesitas un lápiz. - You need a pencil.

     Estoy feliz. - I am happy.
     Estás feliz. - You are happy. (familiar)

However, yo and tú may be used for emphasis.

Yo quiero comer. - I want to eat.