GEDmatch is a great place to collaborate with others who have been tested at other companies and gain access to a more broad audience to try to figure out how you are related to others. Not only will you find more matches, the matches you find will be more likely to have an interest in genealogy than those on other services.
You will need to upload your DNA raw file to make use of the tools. You could download this file from DNA testing services website (23andme or FamilyTreeDNA). Additionally, you can upload your GEDCOM file that is containing genealogical information about your family. You can download this file from family tree builder websites, like myheritage.