Cheapest way to register an LLC ? Which State is best ?

To avoid potential liability, I would like to register my business as an LLC. Can anyone suggest the cheapest way to register a single-member LLC ? I do 95% of my business in Ohio and pay Ohio State taxes however, I'm a resident of NY State. Is an Operating Agreement a must ? Do I need a registered agent or can I just open a PO Box at a UPS store in Ohio and list that as the company address ?