Corp or LLC
If you are going to be the only owner, or a member of a small group of owners, an LLC is typically a good fit as it is easier to maintain and more flexible than a corporation. (You can read our article comparing LLC's to Corporations here).
As every state has different costs and procedures for registering and maintaining entities, it's difficult to be able to say for sure what entity would be the most cost-effective and the best fit for your business plans.
That being said, if you can tell me your state of residence, along with the type of business you would be conducting, I'd be able to give you a much more accurate answer to your question.