Hi Samir!
WAS- is singular
WERE- is plural
Example sentences:
WAS
* I was at the mall earlier this morning.
*He was a perfect student last year.
*Jane was singing when you arrived yesterday.
*She was amazing!
WERE
*They were at the mall earlier this morning.
*Joe and Mary were very competitive.
*The kids were singing when you arrived yesterday.
*You were amazing! (exception with "you", you must use plural)
Have a great day!