Starting a new s-corporation
Forming an entity that would be taxed as S-Corporation from scratch would require you to register it either as Corporation or an LLC with the state where the business would be located/conducted. Once the company is registered with the state, you would obtain the EIN, and then elect the entity to be taxed as an S-Corp with the IRS. In certain states the S-Corp election is required to be filed with the state as well.
Keep in mind, either an LLC or a Corporation may elect to be taxed as an S-Corp, and this election does carry certain limitations. You can read more about the formation and structure of the S-Corporation here.