First Floor of Hogwarts

At the top of the marble staircase from the ground floor is the first floor. The Entrance Hall and Great Hall are several stories tall, so they extend into this floor from below. There are classrooms, offices, and the hospital wing on this floor.

Muggle Studies Classroom
The Muggle Studies Classroom is not really mentioned in the books. People guess it is located here because in the Prisioner of Azkaban on Chapter 16: Professor Trelawney's Prediction, Hermione stopped on the first floor to take a Muggle Studies exam while Harry and Ron continued to Divination.

Defense Against the Dark arts Classroom.
Although there is a new DADA teacher every year, the classroom is always located in the same spot. The room has windows looking out into the ground and it has a chandelier hanging above the ceiling as mentioned in the Chamber of Secrets Chapter Six: Gilderoy Lockhart. From the office in the second floor, there is a staircase leading into the classroom. People say it is on the first floor because of this passage from the Chamber of Secrets:

"(Lockhart) set off back to the castle with Harry, who    was wishing     he knew a good Vanishing Spell, still clasped to his side. 'A word to     the     wise, Harry,' said Lockhart paternally as they entered the building  ...Deaf to Harry's stammers, Lockhart swept him     down a corridor  lined with staring students     and ...They     had reached Lockhart's classroom and he let Harry go at last."

History of Magic Classroom
Professor Binns enters and exits the room through a blackboard. There is a window in the classroom whcih Hedwig once used to enter. As mentioned in Order of the Phoenix Chapter 17: Educational decree Number 24, the window has a thick glass and a narrow ledge. People know the History of MAgic classroom is on the first floor because of this excerpt from the Chamber of Secrets:

Harry, Ron, and Hermione leave the History of Magic classroom,    talk to Colin Creevey,     then     go up a flight of stairs to find themselves on the second floor, where Moaning Myrtle's bathroom is

McGonagall's Office
This office is decorated with plaid. On the desk is a tin of biscuits and on the shelves is the Quidditch Cup. The office also has a frieplace with a mantlepiece, which is connected to the Floo Network as mentioned in the Half-Blood Prince Chapter 18: Birthday Surprises. There is a window overlooking then Quidditch pitch as mentioned in Prisioner of Azkaban Chapter Nine: Grim Defeat. It is close by to Moaning Myrtle's Bathroom on the second floor as mentioned in the Chamber of secrets Chapter 18: Dobby's Reward. People know McGonagall's office is on the first floor from this excerpt from the Sorceror's Stone:

"Filch took them down to Professor McGonagall's study on the first floor..."

Hospital Wing
This room has lots of beds with white sheets, Madam Promfrey's Office, privacy screens, bedpans (as mentioned in the Prisioner of Azkaban Chapter Nine: Grim Defeat), It was moved to the 3rd floor as mentioned in the ORder of the Phoenix. People know it is on this floor in the Chamber of Secrets from this passage:

Harry leaves the hospital wing, intending    to go up to the seventh floor and     the Gryffindor common room. He    gets     to the floor above the hospital wing and     he is at Moaning Myrtle's bathroom.