I want to make a plugin for Unity3d, like TornadoTwins's FPS Control. How can I do that? I already have Visual Studio 2010 installed.
You can find documentation for building Unity plug-ins here:
Do note that plug-ins only work on the Pro versions only, which I assume means you'll be unable to test the plug-in if you have the free version.
More information about creating content on the Unity asset store can be found here.
Depending on what you are making, it may not have to be a native code plugin. Here is the documentation for extending the editor.