The Gnu Image Manipulation Program (GIMP) is an excellent piece of Free and Open Source software, but like most powerful programs, it is complex and can be confusing for the newbie.
There are a few features however, that are very useful for the photographer. I’ll describe them here.
Export vs Save
It is important to remember that GIMP, like many other graphic programs, has it’s own format for saving files. While this is useful for a graphic artist, the casual user should always use the “File / Export as…” feature. This will enable you to use the more common JPG file format.
NEVER EVER WORK on an original image
After you have downloaded all of those wonderful photos from your camera to your hard drive, create a new folder for the ones you’re planning on editing and sharing online or in an email.
Copy the photo you’re going to edit into the new folder, then open the copy with GIMP. If and when you mess it up, you can always go back and copy the original once more.
Let me repeat… NEVER EVER WORK on an original image! 
Change image format
On the upper menu, under “File / Export as…” you can make a copy of the image as a JPG, PNG, GIF, TIFF or almost any other file type. This is handy if you have a very large RAW or BMP image and wish to keep the same resolution yet cut it down in file size by converting to JPG.
Change image size
Most images saved in your camera a very high-resolution in order to achieve sharpness. In most cases, if you wish to share the image on a website or in an email, they are way too big. On the upper menu once more, select “Image / Scale image…” and you can change your photo to whatever size seems appropriate. As a rule of thumb, I generally resize images to no larger than 2400×1800 to share on websites and email. I save the original hires images for paying customers.
Color, Brightness, and Contrast
On the upper menu, select “Colors / Curves…” then click and drag one or two points on the diagonal line to create color curves. You will notice the changes taking place on your image in real time. When you achieve the effect you wish, then click the check button at the bottom right.
Also on the upper menu, under “Colors / Brightness-contrast” you can adjust these values.
Crop image
On the upper menu, select “Tools / Selection Tools / Rectangle Select” or press “R”. That will let you draw a Rectangle on your image. Then, select “Image / Crop to selection”.
This is only scratching the surface of what GIMP is capable of, so don’t be afraid to experiment with copies of your photos. There is a ton of information on the GIMP tutorial page.
And don’t forget to have fun! 
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