Is the name Shinya written as しにゃ or しんや？
In general, if you want to be sure of the correct kana spelling for a Japanese name, your best bet is to find out what kanji it's typically spelled with, and then find the matching readings for those kanji.
In this case, looking up "Shinya" (as a surname), it appears to be most commonly spelled either 新谷 or 新屋. For both of those, the kana spelling would be しん (新) + や (谷 or 屋) = しんや.
Without context, "shinya" would be represented as しにゃ. However, AFAIK there is no name with such spelling so with 99% probability it is しんや. To avoid ambiguity, it is recommended to put an apostrophe between n representing ん and following vowels (i.e. "Shin'ya").