Form1120 or Schedule C or Prepare both C & 1120

A corporation 1120 and a 1040 Schedule C are mutually exclusive.

If they took money out of the corporation, the C-corporation needed to file a W-2 and/or a 1099-DIV to the employee/shareholder.  Then the W-2 and/or 1099-DIV is entered on the 1040.

Because a C-corporation pays its own taxes (unlike a "pass-through" entity like a S-corporation or Partnership), nothing is entered on the 1040 besides the W-2 and/1099-DIV that the shareholder received.

"Completing form federal 1120"

Your question should be stated as: "Completing form federal 1120 for a C Corporation" if the business entity type is a C Corporation.

"for small business first time filer"

Size is not the qualifier for what you should know about this business.

You should find another tax preparer or CPA for your client, or ask that person to mentor you. These are very basic issues you should already have learned. You don't do your best job for this taxpayer if you continue on your own.