Through the software. I would recommend that you go online and look through some Photoshop or GIMP tutorials, because you really need to learn these things if you are going to be making t-shirts.
If you are talking about resizing an image off of the internet to put on a t-shirt, if your image is only a few hundred pixels in height and width (say 480 x 320), then the answer is pretty much "You can't" ...unless you're planning on printing it out at 1.6 x 1.06 inches.