You don't necessarily need to prepare them specially for your fish, but you do need to be aware of how you prepare them for you and evaluate if this is a healthy idea for your fish. For example, I cook my corn and peas with butter. I wouldn't feed that to my fish. But, if you were to open a can of peas and heat em up, there would be no issue feeding a couple to your fish (when cool). Having said that, it is probably best to set aside a little bit prior to preparing food for yourself. This way you make sure nothing "extra" ends up in your tank, or your fish. I imagine there are, in addition to oils and such, seasonings and the like that should not really be eaten by fish.
\Dan