To elect S corporation status, an eligible corporation must file the election of Form 2553 with the IRS. It must be signed by the President (and all shareholders of the corporation as discussed below).
Note that states vary in how S status is to be obtained. For example, states like PA do not require a separate form to be filed to gain S status at the state level. But other states may require a separate state election form to be filed to perfect S status at the state level.
Be sure to check with your corporate or tax attorney as to state law requirements.
Unanimous consent of shareholders is required. All shareholders must sign the Form 2553. Shareholder minutes should reflect this consent and ratify the election by the board of directors and the action taken by officers to effectuate this election.
The Form 2553 must be filed on or before the 15th day of third month of the tax year. As most S corporations are set up as calendar year corporations, generally, this would be March 15.
The election may be filed up to six months from the due date of the original election if there was reasonable cause for failure to file on time. This allows calendar year corporations to make the election until September 15.
For more details as to how S corporations operate please see S Corporations: The Basics.