Learn Portuguese (Portugal): O meu apartamento fica no primeiro andar.

Ordinal numbers must agree with the gender and number of the noun they describe. Because andar (floor) is a masculine singular word, we use the masculine singular form primeiro. If it were a feminine word, like porta (door), it would be a primeira porta.

Why does primeiro come before the noun andar? I thought Portuguese adjectives usually go after.

You are right that most descriptive adjectives come after the noun. However, ordinal numbers (like primeiro, segundo, terceiro) are an exception. They almost always come before the noun they are counting, just like in English.

Why does the sentence use fica instead of está for "is"?

When talking about the permanent location of buildings, addresses, or parts of a building like an apartment, European Portuguese typically uses the verb ficar (to stay) instead of estar. You will often hear this when asking for directions, like Onde fica...? (Where is...?)

Why do we need the article O at the very beginning before meu?