yes i like to use that word too and i saw turkish peaople use it when they unbealive something as turk said.

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all I know is we have two ya´s but it is hard to figure out which is which here

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one popular structure with "ya" is that you can use it for "what if":

ya ... ise = what if ...

ya ... V+ -sa/-se

ya duyarsa? = yes, but what if s/he hears (it)?

ya duysa? = yes, but what if s/he heard (it)?

Some other uses

sen gidiyorsun, ya ben ne yapayım? = You are leaving, well then, what shall I do?

ya, öyle mi? = ah! is that so?

gördün mü ya! = there! you see what happens!

istediğin oldu ya! = after all you got what you wanted!

Niçin söylemedi? söyledi ya! = Why didn´t he say it? He did say!

Öyle ya! = yes, indeed!

ya x ya x (with the same x repeated) it means certainity.

ya gidecek ya gidecek = S/he will either go or go (s/he has to go, there is no other choce)