In America, it's the first floor.

在美国，这一层则被称为一楼。

I live on the first floor.

我住在一楼。

The library is on the first floor.

图书馆在一楼。

The art room is on the first floor.

美术室在一楼。

The first floor has very big windows.

一楼有很大的窗户。