Learn Italian: Voglio una fetta di torta senza zucchero. - I want a slice of cake without sugar.

You would replace di torta with the pronoun ne (which means 'of it' or 'of them' when talking about quantities). Because ne generally goes before the conjugated verb, the sentence becomes: Ne voglio una fetta senza zucchero.

Why is it just di torta instead of della torta?

When you are describing the general material, contents, or type of something, Italian uses di without the definite article. Una fetta di torta just means 'a slice of cake.' If you used della torta, it would mean 'a slice of the cake' (referring to a specific cake you were already discussing).

Italian usually loves definite articles. Why isn't it senza lo zucchero?

While Italian does use definite articles for general concepts, it often drops them after prepositions like con (with) and senza (without) when talking about general ingredients or additions. So, 'without sugar' or 'sugar-free' is simply senza zucchero.