Matching Contacts to Accounts

Hi I am trying to set up the following and getting confused about what data tool I will need to use. I have looked at the documentation and it hasn't really helped me in this area.

I have a Contact with a PIN number. I have a Company with an SFDC Account ID. In the Company record the PIN number can be used to find the SFDC Account ID.

I want to:

1. Look at the PIN number in the Contact field
2. Find the matching Company
3. Add the corresponding SFDC Account ID from the Company to the appropriate field in the Contact record.

Then I can add them to the program that creates new records in SFDC.

Can anyone explain the basic process I would use to do this?

Thanks,

Jonathan