An interesting, though not so easy, project. Upon some googling, I found some projects with an Arduino:
http://www.avdweb.nl/arduino/hardware-interfacing/scale-interface.html
There’s even a crude version right here on the forum: http://community.spark.io/t/coffee-break-anybody-with-maker-kit-publish/4091 A pressure sensitive resistor is being used to sense whether or not the pot of coffee is full.
At first glance it seems as if the pressure resistors aren’t really suitable for this purpose since they’re not really accurate, and mostly don’t have a linear form. Load cells seem to be the way to go. These are the things you’ll find in commercial scales as well. Like @Elijah said: you could either interface with an existing scale (one of the above links), of you could make an entirely new one yourself. It all depends on how much you’re willing to spend (both time and cost), and the skills you have with electronics. Either one should make for an interesting project.
What hardware components do you need? A Spark obviously, then a weight sensor (be it a resistor/load cell/something else entirely; I’ve read about using linear pots connected to springs), some basic electronics (resistors, capacitors, stuff like that), and maybe an amplifier. It all depends on how you’re planning to make it.
A shield is mostly a breakout board which can be easily interfaced with your product by simply sticking it on there. They can contain all sorts of things like sensors, motor drivers, relays, prototyping boards. Well, you name it, and there’s probably a shield for it. They give you easy access to additional functions without having to worry about how you’re going to connect them, you simply plug it in. Would you need one for this project? I don’t think so. Unless you find a shield with a weight sensor on top, and you’re willing to expose your Spark to the weight being applies, there’s not much use for one I think.
The shield shield for the Spark is simply an adaptor for Arduino shields. Since the spark (obviously) has a different dorm factor than an Arduino, you can’t stuff an Arduino shield on it. The shield shield compensates for this by providing a matching layout for the Arduino shields.
What do you need to consider when choosing components? What kind of scale you you want to make, and how accurate would you like it to be? Are you going to measure grams, or tonnes of weight? That’ll certainly make a difference. The accuracy should also be considered. It’s nice if you’re scale can measure between 0-1kg, but if it does so in steps of 500 grams, then it’s not all that useful. Simply put: you’ve got to match the specs with your requirements. (Excuse my metrics system if you’re unfamiliar with it, but I’d like to use meaningful values :p)
Connecting it do the web it the least of you worries thanks to the Spark. First worry about getting it all working, and then just use one of the awesome tutorials provided here to get it on the web. Should be rather easy. If not, there’s always someone willing to help out.
Good luck 
!