Simple tip:
When trying to decide whether to use ser or estar, ask:
Is is a place?
Is it a feeling or state of mind/health?
If it isn't, it should be ser. There are other rules, but if you ask those two questions you will get it right 90% of the time!!!!

Ser is used for characteristics (I am tall, I am nice), proffessions (I am a doctor), to say where you're from (I'm from Colombia).

Estar is used for places (I'm at school, or the party is at school), for mood or state of mind or health (I'm sick, I'm well, I'm sad), for the progressive (I am dancing, I am listening), for things that recently changed (the food is good today, or you are mean today)

The passive voice is used for:
mistery
legal documents
to leave the subject out of a sentence or to put the emphasis on the object

The rules are:
1. There is no subject or
2. If there is a subject, the emphasis is not on it, but on the object
3. The use of the reflexive (there is no subject, and it's conjugated in the third person)
4. The use of ser and/or haber
5. The use of the past participle

Examples:
1. The door was opened (La puerta fue abierta). We don't know who opened the door. There is no subject for the action to open.
2. I was hired in September. (Fui contratada en septiembre). We don't know who hired me. There is no subject.
3. Cars washed here. (Se lavan carros aqui) We don't know who is washing the cars. There is no subject.