Ok I am going to explain how to do this with windows paint because its a standard windows program. If you need to know how to use a different program please post what it is and I or someone else can try to give you specific info regarding that program.
Ok using windows paint, first open the program. Then click "File" in the top left corner, and then click "Open" from the drop down menu. Then use the search window to find the pic you want to work on. Once you have the pic in paint, its very simple to resize it, simply click "Image" from the top middle, then select "Strech / Skew" from the drop menu. You will then see 4 settings, the top 2 are for resizing the horizontal and vertical, the bottom 2 are for pushing the image out of square. using the Hor. and Vert. settings, change the value from 100% to whatever % gives you the size that you want. After you get the image to the correct size, click "File", select "Save As" from the drop menu and save the pic. Voila!! resized picture.
Hope this helps
Steve