There are two methods.
1: cropping
2: resizing
When you crop you are literally cutting out part of the image, which can make the image smaller if you remove the outside.
This can be done easily in paint, or virtually any other image editing software and some none image editing software like MS word.
Resizing is taking your image and changing it by condensing pixels together or separating them out so that the image is larger or smaller.
This can distort the image or cause loss in quality. Once again, there are many types of software able to do this from Irfan, to Photoshop, to even MS word...
If you want a step by step tut. let me know which method, what imaging software you have, and I'll write something up real quick.