Learn Polish: Piję zimne piwo z kolegą. - I am drinking a cold beer with a friend.

The preposition z meaning 'with' (when talking about doing something together with someone) requires the noun to be in the Instrumental case. Because of this, the dictionary form kolega changes to kolegą.

Since kolega is a masculine noun (a male friend), why does it take the feminine-looking ending -ą?

You spotted an important exception! kolega is a masculine noun, but because its dictionary form ends in -a, it borrows its grammar rules from the feminine group. That is why it takes the typical feminine Instrumental ending -ą, becoming kolegą (just like kobieta becomes kobietą).

Why do the words zimne piwo not change their endings if they are the object of the verb piję?

They actually are in the Accusative case, which we use for the direct object of an action (the thing you are drinking). However, because piwo is a neuter noun, its Accusative form looks exactly the same as its dictionary form. The adjective zimne (cold) simply matches it.

How do I know piję means 'I am drinking' instead of 'I drink'?

Polish does not have a separate 'am doing' (continuous) tense like English does. The single present tense verb piję covers both 'I drink' (regularly) and 'I am drinking' (right now). You can usually tell which one is meant from the context.