"Skjorte" is feminine, but all feminine nouns can be inflected as either feminine or masculine in Norwegian Bokmål (that's not the case in Nynorsk). You can use "ei skjorte" or "en skjorte" - that is optional. The choice may depend on your dialect, or it may be a stylistic choice. Many Norwegians (including myself) do not use feminine forms consistently. For a specific feminine word, we sometime use the feminine inflection when we speak and the masculine when we write. And we may use the feminine version for the definite form and the masculine version for the indefinite form (en skjorte - skjorta). And we use the masculine version for some specific feminine nouns, and the feminine for others. I realize that this is confusing for learners, but that's just the way it is.
However, the usual way to ask this question is to put the pronoun at the end - and note that you need the definite form of the noun in that case. In other words, both these options are correct:
3) Hvor er skjorten min?
4) Hvor er skjorta mi?
I would use 4) myself - as in the song from Thorbjørn Egner's classic children's book "Folk og røvere i Kardemomme by
Hvor er buksa mi? Hvor er skjorta mi?
Hvor er munnspillet mitt med fire toner i?
Your sentences 1) and 2) are also possible, especially if you want to stress "my" (Where is my
shirt? I want my
shirt, not his
shirt!). Your sentence 1) is possible also in other contexts, but seems rather old-fashioned, or even stilted. Your sentence 2) would probably not be used in other contexts. It combines "conservative" and "radical" forms of Bokmål in the same sentence, and this combination seems a bit odd.