Sometimes you have to resize a photo for some reason (such as posting a profile pic that needs to be a particular physical size, and usually has file size limits, too).
There are plenty of programs available that let you do that - all the way from Adobe's Photoshop, Elements, Gimp (free), PaintShopPro, Photoscape (free), Irfanview (free) etc down to websites such as http://pixlr.com/ etc. Oodles of choices!
Pick one, download/install it and start getting to know it. Read the help files, read a few tutorials. Resizing is a very basic concept and something quite necessary to learn how to do.
If you only have a little bit to do, then a website will do. If you want to work with photos more often, you're better off with a resident program.
If you need to resize it to a specific size, you will most likely have to first crop it so that the aspect ratio matches, otherwise you will be stretching/distorting the image.
Most programs will let you specify width and height in the crop tool, so that the crop will be restraint to those measurements.
Always work with a copy so you don't ruin your original (or make SURE to save it as a different file name).