If <PERSON>'s parents are Muggles how did she know about Hogwarts in the first place? Someone from Hogwarts came and explained to her and her family(as they do to all muggle borns)
How come her parents didn't wonder about her school? They wilfully admitted <PERSON> into Hogwarts after they were intimated of it. Nowhere in the books or in the films, its shown that they had some kind of issues with Hogwarts or that they didn't know about Hogwarts.
Why she got a letter in the first place ?
There is this concept called the 'Quill of Acceptance and the Book of Admittance' described in Pottermore(offical site of <PERSON>, the author) which states:
At the precise moment that a child first exhibits signs of magic, the Quill, which is believed to have been taken from an Augurey, floats up out of its inkpot and attempts to inscribe the name of that child upon the pages of the Book. No child has ever been admitted whose name has not first been inscribed on the book’s yellowing pages
This is how <PERSON> would have been enrolled and admitted into hogwarts. 899
So when a child whose name is in the book come of age(11 years old), they will get their letter
How she got her letter :
Its never mentioned in the book or in the movies how <PERSON> gets her Hogwarts letter.
BUT
<PERSON> about her admit into Hogwarts in the books:
“Nobody in my family’s magic at all, it was ever such a surprise when I got my letter, but I was ever so pleased, of course, I mean, it’s the very best school of witchcraft there is, I’ve heard –"
-Harry Potter & the Philosophers Stone
To every muggle-born witch/wizard who needs to be given the Hogwarts acceptance letter, a teacher from Hogwarts goes to the kid's parents and explains to the family and the kid about the letter, Hogwarts and the magical world.
Two examples of this:
One -- <PERSON> tells <PERSON> of how she will get her letter
“And will it really come by owl?” <PERSON> whispered. “Normally,” said <PERSON>. “But you’re Muggle-born, so someone from the school will have to come and explain to your parents.”
-Harry Potter & the Deathly Hallows
Two -- <PERSON> goes to <PERSON> to give him his acceptance letter and to explain about magic and Hogwarts.(Harry Potter and the half Blood Prince)
From the above, we can safely assume that one of the teachers from Hogwarts gave <PERSON> her letter and explained to her parents about the magical world 899