The boiling water/wort that your kettle will be filled with will sterilize it. You will never need to worry about sterilizing a boil kettle. Aluminum is perfectly fine to use. You will hear that it should be passivated before your first brew by boiling water in it for a half hour which is easy enough to do. Stainless is more durable and you can use harsher chemicals with it (I've never needed more than hot water for my aluminum kettle) however, aluminum conducts heat better (more efficient heating and ice bath cooling).