Enter a net ionic equation for the reaction between nitric acid and calcium hydroxide.

HNO₃(aq) +Ca(OH)₂(aq) --> H₂O(l) + Ca(NO₃)₂(aq)

It is Ca(NO₃)₂ because NO₃ has a charge of -1, and Ca has a charge of 2+. And when writing an ionic compound, you take the charge of 1 and write it as the subscript of the other.

Now let's balance the equation and then find the net ionic equation.
2HNO₃(aq) +Ca(OH)₂(aq) --> 2H₂O(l) + Ca(NO₃)₂(aq)

2H⁺(aq) + 2NO₃⁻(aq) + Ca²⁺(aq) + 2OH⁻(aq) --> 2H₂O(l) + Ca²⁺(aq) + 2NO₃⁻(aq)

We can get rid of the spectator ions. These are the ions that are on both sides.
We are left with 
2H⁺(aq) + 2OH⁻(aq) --> 2H₂O(l)

We can simplify the net ionic equation down to
H⁺(aq) + OH⁻(aq) --> H₂O(l)