Floor One, First Story, Ground Floor
The one floor is the first floor (or floor one). Pretty simple.
Unless you're British, that is. For you this is the ground floor!
The reading is on'yomi because it's a jukugo kanji. Be careful of the 一 though. It gets shortened and has a little っ in there.
Do you have those snakes on the first floor?
The ground floor is still wet.
"Where should I put my stuff?" "Here, the first floor, the second floor, anywhere. You can put them any place you’d like." "Okay. Thank you."