After you have formed a Corporation by filing the appropriate charter documents, you have a decision to make, largely based on potential tax consequences. Ordinarily, a corporation files its tax returns and pays tax on its profits. When sums are remitted to shareholders as dividends the shareholders pay tax on the sums distributed, amounting to double taxation. Subchapter S treatment is an election that certain corporations can make an election which allows them to be treated like an LLC for tax purposes. There is no double taxation. The corporation's net profit or net loss is deemed distributed to shareholders immediately, just like an LLC. In order to be treated like as a Subchapter S corporation, the corporation must be eligible to make the election the following conditions must exist.
Shareholders: The ownership of an S corporation is restricted to no more than 100 shareholders. The shareholders must generally be U.S. citizens and not corporations and the corporation can have only one class of stock.
Consent: Subchapter S election requires the written consent of all shareholders.
Timing: A corporation must make the subchapter S election no later than two months and 15 days after the first day of the taxable year to elect.
Fees: There is no fee associated with filing for Subchapter S treatment.
The process to make a subchapter S election is filing form 2553 with the IRS. A link to the form can be found here. The instructions associated with filing out the form are found here. The form cannot be filed online.
The IRS has a helpful page with links to other forms you may need if you go the subchapter S route, which can be accessed here.