You can re-use the oil used for frying a few times, just be sure to strain out any food particles. I recommend reading this post from Serious Eats or this post from Epicurious if you have more questions. If you don't feel comfortable reusing it, feel free to discard. To discard the oil, once cooled, carefully transfer it to a disposable container (a funnel helps). Place it in the trash (not recycling). Do not pour it down the sink!