Eu moro no segundo andar.

Portuguese is a “pro‐drop” language: the verb ending (-o) already indicates the subject “I.” Native speakers often omit Eu unless they want emphasis or contrast. Both sentences are correct:

• Eu moro no segundo andar.
• Moro no segundo andar.

No is the contraction of the preposition em + the masculine singular article o.

• em + o = no
  You need it because in Portuguese you usually say “in the [some place].” Here it literally means “in the second floor.”

Segundo is an ordinal adjective meaning “second.” It must agree in gender and number with the noun it modifies. Since andar (floor/story) is masculine singular, you use segundo (not segunda). In Portuguese, ordinals typically precede the noun:

• primeiro andar (first floor)
• segundo andar (second floor)
• terceiro andar (third floor)

Yes. It’s very common in writing—on elevator buttons, building directories and addresses—to use the numeral plus the ordinal indicator:

• 2º andar
  When speaking, however, you would say segundo andar.

In Brazil many buildings have:

• Térreo (ground level, often marked “T”)
• 1º andar (first level above the ground)
• 2º andar (second level above the ground)

English systems vary:

• American English usually calls the ground level “1st floor,” so the Brazilian 2º andar would be the “3rd floor” in the U.S.
• British English calls the ground level “ground floor” and the next level “1st floor,” so the Brazilian 2º andar lines up with the British “2nd floor.”

However, for translation you can safely say I live on the second floor, and then clarify if your listener uses American or British conventions.