How do you write Beth in Japanese?

お名前を聞いて良いですか？ - Onamaewo kiite iidesuka?
It depends on whether you are being formal or informal. If you are being informal, it's 'namae WA nan desu ka' (na-ma-eh WA nan de-s kah) literally 'name is what?'

If you are being formal, simply add an 'o' to the front: onamae WA nan desu ka.

The 'ka' acts as a question mark, so act as if you were asking a question.