Learn Russian: Я покупаю колбасу.

Because it is the direct object of the sentence (the thing being bought). In Russian, feminine nouns ending in -а, like колбаса, change their ending to -у when they are the direct object. This grammatical rule is called the accusative case.

How do you know if покупаю means "I buy" or "I am buying"?

You figure it out from context! Russian doesn't have a separate "-ing" tense like English does. The single present tense verb покупаю covers both regular habits ("I buy") and actions happening right now ("I am buying").

Since we are learning about plurals, how would I change this sentence to say "I am buying sausages"?

You would say Я покупаю колбасы. To make a regular feminine noun ending in -а plural, you drop the -а and add -ы.

If колбаса changes to колбасу when it's being bought, why doesn't the plural колбасы change its ending too?

That is a very handy shortcut in Russian! For inanimate things (objects that aren't alive), the plural form used for direct objects is exactly the same as the standard dictionary plural. So колбасы works as both "sausages" as a subject and "sausages" as a direct object.