Almost all digital cameras are supplied with software for viewing and manipulating the images. Check the help file for references to 'image size' or 'resolution' to find the relevant instructions.
Otherwise, you could download Irfanview. It's a free image viewing and processing program, which should be on everyone's PC:
http://www.irfanview.com/
To resize an image, open Irafanview. Then go to File > Open (or simply click the yellow 'open folder' icon). Navigate to the location of your chosen file and double-click on it. The image will now be displayed in Irfanview.
Next, go to Image > Resize/Resample. This opens a panel with a rather daunting range of options. You might want to experiment with them later but, initially, I simply suggest clicking 'Half', then 'OK'. This will reduce the linear dimensions of the image to half of the original size, so that the area will only be a quarter of the original (with a correspondingly smaller file size).
Then go to File > Save As. Choose a suitable name and location for your new file and click 'Save'.
Chris