After making all possible changes, and when absolutely sure :-) the picture is finished you change the image (colour) mode from RGB to CMYK. As I said you can't do it properly in other programs than in PhotoShop - it's quite a complicated process although you don't need to do anything too hard. In PhotoShop you choose the mode in Image menu. Before that you have to check colour profiles and other stuff but I think you will get help for all of that if/when you win the contest and they ask the CMYK files from you. So don't worry too much about that yet, just enjoy designing! that's my humble advice. I hope this helped you a bit. :-)