You can't change from personal to a corporation. You always have to file a personal Form 1040 tax return. If you own a business that is a corporation, the corporation must file its own tax return, separate from your personal tax return. A C corp files Form 1120; an S corp files Form 1120-S. Your income from the corporation has to be entered in your personal tax return. Exactly how that is done depends on whether it's a C corp or an S corp.
