Learn Polish: Tak naprawdę nie lubię ryb. - Actually, I don't like fish.

Yes! While tak means "yes" or "so" and naprawdę means "really", together they form a very common fixed phrase meaning "actually", "in fact", or "in reality". You can also use the word właściwie to mean the exact same thing here.

Why is the word for fish in the form ryb here?

This happens because of the negative word nie. The verb lubić (to like) normally takes a direct object in the accusative case. However, in Polish, whenever you negate a verb that usually takes the accusative, the object automatically switches to the genitive case.

Is ryb singular or plural in this sentence?

It is plural. In English, the word "fish" is the same whether singular or plural, but Polish treats ryba as a standard countable noun. Because you are talking about fish in general, you need the plural form. The plural genitive of ryba is ryb. If you meant you did not like one specific fish, you would use the singular genitive: Nie lubię tej ryby.