I'm going to assume you are using jpeg files.
You'll need a software program to resize your photos. Photo editing software like Photoshop have that feature built in. There are a number of them available, some you pay for and some are free. If you download one of these programs then you can also do some basic editing like sharpening, colour adjustment or fiddling with exposure.
I use Lightroom to edit and resize most of my work. However I also use a freebie called Image Analyzer for some stuff.
However if all you want to do is resize and you are using Windows then you can use Paint. Its usually preloaded on your computer.