How do you build a mini bike?

If you're a first timer, get a mini-bike kit. It will be a fun project for you, if you enjoy tinkering with mechanical stuff, and you will learn lots of new things. Northern Tool has a kit (http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200006624_200006624) and they also have a wide array of engines you can order for it.

Have fun.