Translation of ya is not that easy to understand when you do it literally. But if you keep in mind the meaning, then everything become easy to understand not matter the situation. Ya means already or now. If you say "Ya voy", it means I go NOW, if you say "Ya saben que estamos aquí", then you can think as, Before they didn't know we were here, but NOW, they know we are here, of course the shorter version of this is, They already know we are here. So what you need to do is just read some few examples of this and get use to it. Here I give you some examples. Quiero comer ya. Ya no tengo dinero. Ya Obama no es presidente. Ella ya no es mi novia. Ya es de noche. Ya no la amo.

It is like Daniel said and it is also used in fixed expressions, for example: "ya es hora" = "It's about time!"