When you hire a new employee, the I-9 is the form the employee should complete that proves their eligibility to work in the United States. As the employer, you will not be penalized for hiring an unauthorized worker if you have acted in good faith and documented your efforts to make sure the worker was authorized.
Regarding the I-9, you should have your workers fill out Section 1 of the form on their first day of work and provide the documents proving their authorization to work by the third day of their employment. Once all of the appropriate information has been provided, complete the employer section of the form.
Keep the I-9 on file for at least three (3) years from the date of hire or one (1) year from the date of termination, whichever is longer. For safe-keeping, you can populate the information from the I-9 and load an electronic copy into your iSolved database by going to Employee Maintenance > Personal and clicking the "I-9" tab.