DC motor speed control

hi

This is muralidhar.i am planing to create an electrical bike for that
i am planing to use a dc motor
(12V) but i don't no how to control the speed of that motor since it
is a DC supply i cant use transformers. I have little knowledge about
DC supply because i am electrical engineer  EEE
now i am in IT field so i don't have clear idea.


can any one help me what are the simple way to vary DC voltage with
some digram which can be made in home .(the thing is i should handle 4
to 6A current)
or
any other solution to control the speed of that motor

please send me mail on below address

id:admin.murli@[EMAIL PROTECTED]