The word ya usually means "already":
Ya tengo los billetes de avión – I have already got airplante tickets
But it also means "right now". In this case it should be pronounced with a stress:
Voy a salir ya – I´m going out right now
Quiero que recojas los juguetes ya – I want you to gather up your toys right now
But it can be used in many other cases. First of all, to show solidarity or to admit the other person is right:
– Estoy muy cansada. Claro, con dos niños pequeños... – I am very tired. Of course, with two little children...
– Ya – I see / I know
– Te veo un poco preocupado – You seem worried
– Ya, es que mañana tengo un examen = You are right, tomorrow I have an exam
The same word is used to stop putting you more and more food or drink:
– ¿Unas patatitas? – Some fried potatoes?
– Muy bien... Ya – Great... It´s Ok, don´t put me more
And to show we have just done a work:
– ¿Hablamos? – Can we speak?
– Sí, espera un momento... Ya = Yes, wait a minute... Done!
Least, but not last, it´s frequently used with verbs saber and ver adding a kind of firmness or stress:
Ya sé que es tarde – I perfectly know that it´s late
Ya sabes que te quiero – You know perfectly that I love you
Ya veré qué voy a hacer – I´ll arrange it by myself
Ya nos veremos – See you later