How to add external power supply for drivin motor?

Hello, I want to add a 12V DC motor to arduino. For driving the motor i require external power supply of 12V and need arduino to control the motor speed etc. In fact i would like use arduino as switch. Would you please tell me how is it possible? Also, how can i add up other power consuming devices and using arduino as switch and as controller? Thank you.

See this page:- http://www.thebox.myzen.co.uk/Workshop/Motors_1.html

and this one:- http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

Thanks a lot. :)