"Were you scared?"
Here, you're using 'scared' as an adjective. Adjectives in their base form are 'to be tall', 'to be scared', 'to be angry', 'to be surprised'. They need the verb 'be'. Therefore your short sentence is correct as 'WERE you scared?' (Because 'were' is the past form of 'be' conjugated to 'you')
There is often a lot of confusion about what is an adjective and what is the passive voice, because, with regular verbs, it often looks the same. (The past participle is often used as an adjective - e.g. He is frightened; She was interested; They are excited, etc.)
'Did you scared' is incorrect.
To use 'scare' as a verb in the past, we would say 'Did he/she/it scare you?' (Here we need the helping verb 'do' to create the question. All questions in English need a form of 'be', 'have' or, most commonly, 'do' to 'help' them.)
Although your question was a short one, learning about these things will take a lot of time and practice, and structured lessons might help you.
I wish you lots of success in your English learning.