All previous answers are correct, but also consider that the floor on the ground level is called "Planta Baja". The one on top of it is "primer piso", and so on... And underground is "subsuelo".

first floor = primer piso second floor = segundo piso third floor = tercer piso fourth = cuarto fifth = quinto sixth = sexto seventh = séptimo eigth = octavo ninth = noveno tenth = décimo :)

the first floor = El primer piso the second floor = El segundo piso Primero Segundo Tercero Cuarto Quinto Sexto Séptimo Octavo Noveno Décimo Décimo primero etc.