Your camera almost certainly came with some software which can do the job for you. The more recent versions of Windows also have a suitable facility built in.
However, here's a piece of (free) software, which deserves a place on everyone's PC:
http://www.irfanview.com/
It's a superb image-viewing program but, if you delve into its options, it can do much more (including what you want to do).
Open an image in Irfanview.
Click on Image > Resize/Resample
Choose a suitable size from the panel on the right. (If the picture is only to be viewed on screen, rather than printed out, 640 x 480 (or, at the most, 800 x 600) is good enough. (Alternatively, choose a suitable percentage figure from the panel on the left).
Click 'OK'
Click File > Save As
Ensure that 'Save as type' is set to 'JPG'
Choose a slightly different file name (e.g. 'SueOnBeachX', instead of 'SueOnBeach')
Click 'Save' and then send the modified file.
Chris