Question

How to use: was and were??

Answer 1

I was
You were
He/She/It Was
We were
You Were
They Were 

Exceção: 
Se o que você quiser dizer tiver o sentido de Se eu fosse ..... eu teria ... 

Nesse caso não dirá If I was .. mas sim If I Were ... 
Lembre-se da música da Beyoncé - If I Were A Boy 

Answer 2

Voce usa Was/Were para indicar ações no passado:

We were at the club yesterday (Nós estávamos no clube ontem.)
Use WERE para I, you, we, they.

Use WAS para he, she, it.

I was at the bank last night. (eu estava no banco ontem á noite)

Pode usar no Present Continuous > I was reading a good Spanish book. (eu estava lendo um bom livro espanhol)

Were you calling me? (Você estava me chamando?)