OK, so this is technically not a color issue, it is a lack of color, but it is a cool effect so let me walk you through the simplest way to make your color image into grayscale, which is the technical term for black and white. Click on 'Image' then click on 'Mode' and finally click on 'Grayscale'. Your photo is now B&W.
Now you can go any enjoy playing with the colors in your photos, and stay tuned to this series for more on how to make yourself into a GIMP master.