In Standard English, were is the only form used with you. "You was" is ungrammatical in Standard English.
You must be confused because you must have seen both "I was" and "I were". Now that is a separate story, which has to do with the so called irrealis.
If I/he were a bird, I/he'd fly all day. //irrealis
I/he was a very noisy child when I/he was young. //indicative mood.
However, with we/you/they, "were" is used in both cases
If you were a bird, you'd fly all day.
You were a very noisy child when you were young.
Do note that in certain non-standard dialects, notably the African American Vernacular English, "you was" is possible
Yo, what's poppin'? Tell me why you was so angry with me yesterday.