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:-

and this one:-

Thanks a lot. :)