Hello friends here,
(Today) refers to the day which we omit and use (it) instead ,so it is the subject , this means that today is hot or cold is the right one unless there �s a comma to separate today from it , so we say (Today, it is hot ) so today here is just expressing time and (it )is the subject pronoun
Today it �s sunny (This is wrong ,no comma ,repitition of today and it)
it �s sunny today (right)
is it sunny today? (right but is should have a capital letter )
is sunny today? (wrong ,there is not subject)
is it today sunny?(wrong ,today should only be at the begining or at the end)