Questions like yours make me quite sad. A PhD candidate should be a person highly knowledgeable in his/her science and exactly knowing what he-she intends to do to make a breakthrough in his/her science field.
Anybody not being ready should first go to work and study hard by nights to get ready. Anyone who needs such advice is simply not ready and will fail.
A PhD thesis suppose to be something nobody did it before. I wonder if Edison asked somebody what inventions have to think about. He was a "school dropout boy", but this doesn't stopped him to hold 1093 US patents.