First floor, ground floor

As far as I know, it depend on whether you're in the USA or in the UK

Here's an excerpt from the OALD:

In BrE the floor of a building at street level is the ground floor, the one above it is the first floor and the one below it is the basement, or lower ground floor in a public building.

In NAmE the floor at street level is usually called the first floor, the one above it is the second floor and the one below it is the basement. In public buildings the floor at street level can also be called the ground floor.