From William Perez,
Your Guide to Tax Planning: U.S..
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Preparing the S-Corporation tax return, IRS Form 1120S, is not hard, but it is not easy either. If the company has accurate bookkeeping, then preparing Form 1120S can be straightforward.

However, I highly recommend hiring a tax professional to prepare a company's tax return. Why? Tax professionals such as Enrolled Agents and Certified Public Accountants are trained to analyze a company's financial statements looking for a complete and accurate picture of a business. A tax professional is also attuned to the latest IRS audit and compliance issues that an S-Corporation is likely to face.

Here's the minimum you need to prepare an S-Corporation tax return:

• a complete set of financial statements: Income Statement & Balance Sheet,
• a complete transaction history and summary statement of each shareholder's capital accounts,
• tax software that prepares 1120S tax returns.
• complete information about each shareholder: name, address, and SSN.

In addition, you may want or need additional financial records, such as:

• payroll tax returns,
• copies of W2 statements,
• sales tax returns,
• transaction details of federal, state, and local taxes paid.

The deadline for filing Form 1120S is March 15th, or the 15th day of the third month following the end of the corporation's tax year. (S-Corporations almost always must have a calendar year.) The corporation can request an automatic 6-month extension using IRS Form 7004 (PDF). The extended deadline for corporate returns is September 15th.

More information: Preparing Schedule K-1 for IRS Form 1120S