When you introduce something new, you use
det er regardless of the grammatical gender of the noun. After that, you can use
det (neuter) or
den (fem./masc.) as a pronoun that directly replaces and refers to the object, which is now no longer unknown to the reader/listener.
Sko is indeed a masculine noun, so I'd expect a longer description to sound something like:
DET er en sko. DEN er min= That is a shoe. It (where "it" directly replaces "the shoe") is mine.
As a refresher,
en (masculine), and
en (neuter) all mean "a(n)". One decides which is the correct article to use according to the grammatical gender of each noun.
Nowadays, feminine nouns like
bukse are often treated ("declined") as masculine, so you see phrases like
ei bukse less and less and phrases like
en bukse more and more.
That being said, both forms are grammatical, and there are even some "strong" feminine nouns that tend to retain their feminine declension, e.g.,
jenta usually being preferred over
jenten. It all boils down to dialectal differences! ;)