Entrepreneur has a page that lists the costs of becoming a car rental agency franchise. Above all, try to be process oriented and methodical no matter what type of business you start. There's a pretty well documented tried and true process on how to start a business in a step by step way. Here's a solid resource on how to start a business that applies to either online or offline businesses: http://sellitontheweb.com/blog/start-web-business/ IRS website has a great checklist for starting a business and the Entrepreneur website also has a great page on how to start a business as well.

A lot of what you ask depends on where you are located. Contact your Secretary of State's office - they should be able to answer your questions. Or, find a small business development center (SBDC) - free service - and let them help you put together a business plan. Lastly, if you don't have experience - then go work for a rental company for a while - this way you can learn the business inside and out - including what you need to get started.