The link provided by sumelic is a very good one with some great rules.
However, I want to point out that the simplest rule to follow is if your object is plural then you would use "were". If the object is singular then you would use "was".
That being said, and even in the link provided, the example sentence of, "I was always delighted when my brother or one of my sisters was asked to do them.", would not be misinterpreted or look strange to native speakers if you swapped it for, "were". In our vernacular it's common to use either.
The main thing is you never use "was" for an obvious plural subject or "were" for an obvious singular subject/object.
"We all was running around the yard having so much fun playing." - WRONG
"We all were running around the yard having so much fun playing." - CORRECT
"My brother and one of my sisters was attending church that day." - TECHNICALLY CORRECT
"My brother and one of my sisters were attending church that day." - STILL COMPLETELY ACCEPTABLE IN SPOKEN ENGLISH
"My brother and my sisters was known for going to church every Sunday." - WRONG
"My brother and my sisters were known for going to church every Sunday." - CORRECT
Just remember that if there is any question at all if the subject is plural then you should likely use "were". If there's anything at all that makes it appear singular then you should probably use "was".