Learn French: Mon ami est américain. - My friend is American.

In French, nationalities are only capitalized when used as a noun to refer to a person (like un Américain, meaning "an American person"). When used as an adjective describing someone—as it is in this sentence—it stays lowercase.

If the friend is female, would the sentence start with ma amie?

No, it would actually still be mon amie! French doesn't like two vowels bumping into each other (ma ending in a vowel, and amie starting with one). To make it smoother to pronounce, you use the masculine mon before any feminine word that starts with a vowel or a silent h.

How does the rest of the sentence change for a female friend?

It becomes Mon amie est américaine. You add an -e to ami to show the friend is female, and you must also add an -e to the adjective américain so it agrees with the feminine noun. Adding this -e changes the spoken pronunciation, meaning you clearly pronounce the "n" sound at the end of américaine.

Should I pronounce the "n" in mon when reading this sentence?

Yes! Because mon usually has a nasal sound with a silent "n", but ami starts with a vowel, you link the two words together when speaking. This is called a liaison. It makes the two words flow together and sound almost like "mo-nami".