Controlling 12v with a transistor

Hello all, I am starting my first ever project and need some help,

I am trying to switch 12v with a transistor through arduino, I have tried reading about it through google etc but am struggling to work out what I need to do, I have managed to switch an led strip through a transistor but I was switching the ground, for my project I want to have a permanent ground with a switched 12v supply.

Please could someone explain to me what I need to do to switch 12v through a transistor using arduino? Please explain in very simple terms as I am a complete novice, an accurate parts list would be great so I can get what I need, from what I've read I think i will need a MOSFET transistor but please correct me if I'm wrong as I am here to learn.

Thanks in advance

Ps, I want to switch about 30W

Here is a short tutorial on using a transistor to switch power to a load.
http://techhouse.brown.edu/~dmorris/projects/tutorials/transistor.switches.pdf

Bob Weber