If you mean to cut out the parts of the image which are not relevant. The easiest way is to try an online service like www.iloveimg.com. You click to select your photo from the computer and just drag the box to the area you want and then click download and it will download the reduced size image.

If you have the photo on your phone, thought just Google "how to crop image on *Android/iOS*" There will be something with a visual directions how to do it.