The アメリカ part is pretty easy so long as you know katakana. It's the word for "America." You know that 人 means "person," so you can figure out that America + Person = American.

Often times when two kanji come together to form a combo kanji you use the on'yomi readings. This is similar to that, though instead of another kanji it's a foreign word. So, the reading of 人 is the on'yomi reading じん, which you learned already in the kanji. This pattern works with almost any location as well. Just add a country name + 人 and you've described the people who live there. You'll see more of these before you're finished here.