See answers to your questions inside

1。 是不是有不超過30天免美國簽證一說？ 需要出示些什麽證件？ 我是學生。-- You don't need US visa if you come back within 30 days. Bring all of the documents related to your status in US, and of course, all of your passport (if you have changed your passport).

2。 想開車去。可是目前我的車子是這邊一個親戚幫忙貸的款 (我自己信用不夠好)，所以車主是他，不過是我開著。如果我想開車去，是不是要是自己的車才行？ 而且必須擁有這個車半年之久？ 對嗎？ -- You can drive any car, including renting a car, as long as you have driver license and auto insurance.

Good Luck!