The easiest way to convert your bike to electric is to install a hub motor kit. Just go to this link.
http://www.werelectrified.com/unibikekit.html

A better choice may be to order a complete electric bicycle. Wal-mart has a super deal on an electric bicycle. It has the same 450-watt motor as your skateboard on a bicycle designed to have electric power. Best of all this electric bike only costs $279.46!
http://www.walmart.com/catalog/produ...uct_id=5391437