Well, it largely depends on what you want your plugin to do… I mean, plugins load before most things, and so they can’t make any real output on their own. They use action hooks to change the output elsewhere.
In other words, I need more information, your question is too broad. What, exactly, are you trying to do? Be specific.
Anyway, I suggest you read up on the Plugin API. If you don’t know actions and filters, you can’t write a plugin.