Firstly, take a picture of yourself, in underwear or swimsuit. Make sure you stand and look squarely to the camera. (Sorry, no pictures of this step haha :D)

Now, look for some cool clothes you'd like to wear. e.g. delias.com has pretty much clothes with white backgrund. Dowload some of them.

Put your pictures and the clothes to photoshop. With the magic eraser tool, erase the white background of the clothes, so now you can put them on your body.

You can mix & match the clothes, it's fun and it can help you to spend less time in the crowded shopping malls. Also, you can find out easily what clothes look good on your body type (I am pear shape, so A line dresses & skirts look good on me)
Hope I helped, if you don't understand something please tell :)