Let's begin with a notion: compressors are able to reduce the size of something (like a text file) by processing it and representing the same information using less symbols. It's the way Zip makes a text-file smaller. This is lossless compression.
Pictures and audio (and, so, video) uses a very big amount of information: for example, a 5 megapixels picture, in a format that does not compress anything (like TIFF) would take 5 megabytes x 3 colors *RGB) = 15 MB of space.
To reduce the file size, JPEG format makes you loose some information from the picture: be it quality, colors, etc. That's why you can open a JPEG in some photo-editing program (Photoshop, Gimp, etc) and then save it with less quality: it will use less information to represent your image, with a smaller file size, but will loose some quality. If you (or any person) will be able to notice the low quality is a important point when choosing "how much compression" you want.
So: in general, for practical purposes, you will loose a lot of quality to reduce the size of a JPEG picture, because it is already compressed.
But you want to use that in your cell phone, that's right?
You didn't say which cell phone you have. Let's assume that you have a Samsung galaxy s3. You can see it's specifications: the screen resolution is 1280x720 pixels.
So, when you view a picture in it, it doesn't matter the original picture size: you'll be able to see 1280x720 pixels, because that's the maximum amount of pixels that the cell will be able to show.
If you take all your pictures, resize them to 1280x720 pixels, and open in your phone, you will see them the same way you would see them with the original resolution. You will be loosing "resolution" from the original image, but it's something that you wouldn't be able to see in your phone anyway...
How yo do that in Windows: use any program you like that is able to edit pictures: Photoshop, Gimp, IrfanView... but since you're talking about doing it in a large amount of files, probably you're thinking about ImageMagick or a similar program, that can perform the same operation in a large amount of files.
Edit: to do it with ImageMagick:
1 - copy all your pictures to a new folder (ex.: Temp1), to avoid loosing the originals
2 - create a new folder where your pictures will be after being resized (ex.: Temp2)
3 - open a
cmd window, go to the directory that have the copied pictures (Temp1)
4 - type:
mogrify -path full_path_to_Temp2 -resize 1280x720 *.jpg