I've gone through the basics of image resizing using Photoshop (which you should read first, as the fundamentals are the same) however, if you don't have Photoshop there are a few online tools that you can download to resize images.
Paint.net is a relatively simple tool to use but you have to make sure once you have opened your image in Paint.net that you are zoomed in to 100% before doing any editing.

Once you have done that, go to the tool bar and click image > Resize.

Once you've changed the resolution and width/height dimensions save your image as desired.

Another way of editing your image is by using Picasa.
Once you've downloaded Picasa click 'Export' at the bottom of the screen then click image quality> custom (85) and save as desired.

The nice thing about Picasa is that it allows you to edit your image. If you need to crop, add contrast or sharpen your image there are a number of effects you can use to enhance your photo.

If you don't want to download any programs like Picasa, there are a number of online tools you can use like Picresize, which will allow you to resize an image or a batch of images if you have a number of photos to edit.