Linda's suggestion below is a must. You may also want to view this page with some tips, hints and answers to some questions that may pop up when using GEDCOMpare:. GEDCOM File Usage Primer.
Current_GEDCOMpare_Process is an excellent how to do it work page while this page GEDCOM_File_Usage_Primer is more of a why it works page.
It would be a good idea to review the Current GedCompare Process which will explain the process. It is advisable to have a very small Gedcom initially, so you can learn the process. You will not be able to add any records to wikitree until you have compared all records in the Gedcom first.