Learn Polish: Po południu piję kawę. - In the afternoon I drink coffee.

Literally, it means 'after noon'. The word po means 'after', and it forces the noun południe (noon) to take the Locative case, changing its ending to południu.

Why does the word for coffee change to kawę here?

The dictionary form is kawa. Because the coffee is the direct object of the verb (it is the thing being drunk), it takes the Accusative case. For feminine nouns ending in -a, you just change the -a to -ę.

Doesn't the word południe also mean 'south'? How do I know which one it means?

Yes, it means both 'noon' and 'south'. However, po południu is a fixed phrase that always means 'in the afternoon'. If you want to say 'in the south', you use a different preposition: na południu.

Why isn't there a word for 'I' (ja) in this sentence?

The verb ending does the work for you. The -ę at the end of piję tells you it's 'I drink'. You only need to include the word ja (I) if you are comparing yourself to someone else or adding special emphasis.