The spectator ions would be the ions that don't comprise the precipitate in this reaction. You have four ionic species here: Pb²⁺, NO₃⁻, H⁺, and SO₄²⁻. Since Pb²⁺ and SO₄²⁻ combine, as you are told, to form the precipitate, that leaves H⁺ and NO₃⁻ (or, in normal text, H+ and NO3-) to be the spectator ions.
Edit: NO3- might be interpreted ambiguously since it's not immediately clear that the NO3 is a polyatomic ion with a -1 charge and not an NO compound with a 3- charge, so it should be written as [NO3]- or (NO3)-.
The net ionic reaction shows the reaction without the spectator ions. In this case, that would be: Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s).
