Everyone except sole proprietors who have no other employees are required to have a federal employer identification number. To obtain a FEIN, download and fill out Form SS4 from the IRS (pdf). Then call the IRS at (800) 829-1040 to review the form with them and request a federal ID number. If you are in the northeast US, call (978) 474-9714.
Publications you may find helpful are:
If you are a sole proprietorship, partnership or LLC organized as a partnership, you must pay Self-Employment Tax. This is the equivalent of social security and Medicare for people who are self-employed. It amounts to 15.9% for 2001. You must use IRS Form 1040-ES to make your quarterly estimated taxes. If you have incorporated, you will need to make estimated tax payments on Form 1120-W, Estimated Tax FOR Corporations (PDF).
There are commercial web sites that will provide tips on how to estimate how much to pay. Your tax advisor will also be a valuable resource. Contact the Social Security Administration for a definition of self-employment, or for information on estimated taxes.
By January 31 or within 30 days of termination of employment, you must furnish copies of Form W-2 to each employee to whom you paid wages during the year. Additionally, you must also send copies to the Social Security Administration by February 28 for non-electronic filing (or April 1 for electronic filing).
See IRS' Small Business and Self-Employed Community for more information.