How can I build a touchpad/trackpad?
There are tons of external touchpads on the market, from the giants like Wacom, Logitech or Lenovo to cheap no-name ones. I doubt you'd be able to approach their precision in home-made device. Some of them are pretty big (and expensive).

If you still want to make one yourself you can start looking in two directions.

1) find capacitive sensor and figure out how to interface with it from something like Arduino. This Snowpad project is good example, but it does not look very "open source" anymore.

2) learn how to communicate with computer via USB. What you need is to implement 
something called HID class device.

Oh, and if you want to get really fancy, maybe blackberry trackball would be more to your liking.