Complete Noob, need advice

Hi there, I'm new to arduino, I understand slightly the methods used for ethernet shield applications, but i need some help. I want to make a device that is connected to a tripwire over the back of my letterbox, when the flap opens the wire is tripped and the device sends me a tweet on twitter, telling me I have post. From my undestanding I need an arduino duemilanove thingy, an ethernet shield, a laser diode, mirror, a ldr . Could someone guide me to what else I need, such as what power source to use? And once I have linked it to my pc, how do I programme it to tweet to me? Sorry for being a noob, but a simple blinking led light as a project doesn't really fill me with joy -.- p.s i am in the uk and use a windows 7 pc

You will need an Ethernet cable between the Ethernet Shield and an Ethernet port on a router connected to the Internet. Power can be supplied over the Ethernet cable if you get the optional Power Over Ethernet (POE) module on your Ethernet Shield and your router can supply POE. Otherwise you should probably get 9V ‘wall wart’ power supply that can plug into the power jack on the Arduino.

Look at the Files->Examples->Ethernet->TwitterClient example.

And maybe for ease of building you could use a simple push button that changes state (goes from high to low, or low to high) when you open the mailbox door. that would be a little easier than laser diodes and ldrs, both in construction and programming. Look at Files -> Examples -> Digital -> Button example. Love the idea!