First, you need a balanced equation (including products):
Ni(NO3)2 + 2NaOH --> Ni(OH)2 + 2NaNO3
Now, check your reactants and products for solubility:
Nitrates are always soluble, and hydroxides are usually insoluble (exceptions are NH4OH and Group 1A hydroxides). On the products side, we have nickel (II) hydroxide and sodium nitrate, and since nickel is an insoluble hydroxide, it will precipitate from the solution.
Now, write a complete ionic equation by separating the ions of each reactant and product:
Ni^2+ + 2 NO3^1- + 2 Na^1+ + 2 OH^1- --> Ni(OH)2 + 2 Na^1+ + 2 NO3^1-
Cancel the ions that appear on both sides of the equation (spectator ions) to leave the net ionic equation:
Ni^2+ + 2 OH^1- --> Ni(OH)2