Which season are you beginning to watch?
I began watching Supernatural from season 4.
I want to ask the others this question, but I don't know how to say.

Which season are you beginning to watch?
Which season do you begin to watch?
Which season do you begin with?
Which season do you begin watching?
Which season do you start from?

Is there any correct sentence?

Question

Which season are you beginning to watch?
I began watching Supernatural from season 4.
I want to ask the others this question, but I don't know how to say.

Which season are you beginning to watch?
Which season do you begin to watch?
Which season do you begin with?
Which season do you begin watching?
Which season do you start from?

Is there any correct sentence?

Answer

Since these sentences should be referring to an action in the past, you should be framing the questions in past tense.

Which season have you started to watch?

However these questions are also a little unnatural to me. Here are some questions you might ask them instead.

Which season are you on now? 
Have you started watching Supernatural yet?
Which season are you watching?

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