Generally, you do not use a comma before "or" unless it is used to separate two independent clauses.
Here are some examples:
No comma needed:
* Would you like tea or coffee?
* She is a doctor or a nurse.
* I will go to the park or the beach.
Comma needed:
* I went to the store, or I would have been out of milk. (Two independent clauses)
* She was tired, or perhaps she was just bored. (Two independent clauses)
Here's a simple rule of thumb:
* If you can replace "or" with "and" and the sentence still makes sense, you likely need a comma.
* If the sentence does not make sense with "and," you likely do not need a comma.
Let me know if you have a specific sentence you'd like me to help you with!