I will be making a heart shaped cake for valentines. What can I use to stick the fondant to the cake like buttercream or ganache? Can I use something ready made like Nutella for example?
I don't think Nutella alone is not suitable. You want something that can be smoothed nicely. If you're not very experienced try American buttercream, that's super easy to make and can be flavored with Nutella.
In general you should use any buttercream or ganache under fondant if you want to cover an entire cake. If it's just fondant letters or little fondant hearts you wanna add on top you don't have to worry. They can be stuck onto about anything, even placed on a "naked" cake.
As julia said any b'cream or ganache can be used BUT............the fondant needs to be put over it as soon as you finish icing it. Don't let it dry before trying to cover w/fondant. I have never tried using Nutella so can't speak to that part of your ?.