See answers to your questions inside

1。 是不是有不超过30天免美国签证一说？ 需要出示些什么证件？ 我是学生。-- 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。 想开车去。可是目前我的车子是这边一个亲戚帮忙贷的款 (我自己信用不够好)，所以车主是他，不过是我开着。如果我想开车去，是不是要是自己的车才行？ 而且必须拥有这个车半年之久？ 对吗？ -- You can drive any car, including renting a car, as long as you have driver license and auto insurance.

Good Luck!