I want to be able to measure weight (typical range of human weight), what sensor can I use?
Like this? http://www.walmart.com/catalog/product.do?product_id=5740778

You could buy one and interface to its strain gage.  Or even its measurement electronics.

If you are making a computer interface to a scale, try to stick to the SMA Serial Communication standard: http://www.scalemanufacturers.org/smastandards.htm Then your scale will be instantly compatible with all software.