Working with Variables
Within your variables class file you will want to define methods that 'return' values to your template, like so:

class HelloVariable
{
    public function message()
    {
        return "Hello, world!";
    }
}


Which you would call from your template using:

{{ craft.hello.message }}


On a side note, a really good place to start plugin development is to download and install a plugin template such as BusinessLogic, which provides a basic plugin structure, some sample files, and includes inline documentation for basic business logic kind of functionality.