Use EC01 to copy an existing company code.  This will ensure that you will not missed out any customizing settings.

OKKP - Assign company code to the controlling area
OBY6 - Define company code golbal parameters
OB62 - Assign company code to chart of accounts

Then check the following if needed, there may be other areas depending on your needs:

OBA4 - FI Tolerance Group for Users
FBZP - Maintain Payment program
OBYA - Inter Company code clearing if being used

List of Important FI Tcodes
Useful SAP FI Related Transaction Codes

SAP FICO Reference Books:
SAP Financial and Controlling Interview Questions, Certification and Configuration, Books

Goto SAP CO Tips:
SAP Controlling

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SAP FI (Financial Accounting) Configuration Hints and Tips

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SAP ABAP/4 Programming, Basis Administration, Configuration Hints and Tips