1- Keep in mind that most rental companies will not rent to any driver under the age of 25. If you fall under that magic number, you more than likely will need to get an older relative or friend to sign the actual contract for you.
2- You will of course, need a valid drivers license to rent the car. If you do not have a license, they will not rent to you under any circumstances. Most companies will also require you to reserve your car with the use of a major credit card, however, you can pay in cash if you would prefer.
3- You do not need to have personal auto insurance to rent a car; they will ask you if you would like to purchase their insurance for the duration of the rental contract. If, however, you have your own insurance policy, be sure to call your agent to see if you are covered during your use of a rental car. If your personal auto policy covers you, there is no need to purchase a policy from the rental company.