Whether you're looking to expand your ability with coins or just trying something new, there are SO many different options available for you! The possibilities are understandably intimidating, so how can you decide which is right for you?
Before doing ANYTHING, ask yourself "What type of shell do I want?"
-(Regular) Shell: A normal shell is a coin made with one hollow side and one "clean" side. The coin is made to be the exact size as the coin you're trying to match. For example, a half dollar sized shell will be the exact same size as another half dollar.
-Expanded Shell: An expanded shell fits the same description as your regular shell, except the coin is slightly larger in size to the point where you can fit the shell on top of the coin you're matching. For example, a half dollar expanded shell will fit over a regular half dollar and will look like one coin.
Now that you know what shell you need, I recommend checking out Troy Hooser's "ShellRaiser" DVD: http://www.penguinmagic.com/p/1996
This DVD is fantastic because it teaches some high quality routines using a shell that ANYONE can do, but also includes a beginner's double sided coin shell. A double sided coin shell looks the same on the clean side and the hollow side.
Once you're ready, you can move up to a more high caliber shell, such as Tango or even get one made specifically for your own needs by Lassen or Schoolcraft.
As a final note, be sure to check that your country's currency is available in a shell model!