Learn Portuguese (Portugal): Estou na paragem de autocarro.

In Portuguese, we use the verb estar (which becomes estou for "I am") to talk about locations and temporary states. Since waiting at the bus stop is a temporary location, we use estou. The other verb for "to be", ser (like sou), is used for permanent things like your name or nationality.

What does na mean, and why isn't it just em?

na means "at the" or "in the". It is a mandatory contraction (combination) of the preposition em (in/at/on) and the feminine word for "the", which is a. We use na here because paragem is a feminine noun. If the location was a masculine noun, we would use no (em + o).

Why is it paragem de autocarro instead of just "autocarro paragem"?

In Portuguese, you cannot stack nouns together to describe something like we do in English with "bus stop". Instead, you have to connect them with the word de (of). Literally, you are saying "stop of bus".

I've seen the word "ônibus" used for bus online. Why does this sentence use autocarro?

autocarro is the standard word for "bus" in European Portuguese. The word "ônibus" is used in Brazilian Portuguese. Also, a bus stop in Portugal is a paragem, whereas in Brazil you might hear "ponto".