How to Save Customized Settings

Hi,
      I'm trying to develop a plugin for IDEA. I created it as a tool window so it can be accessible in view -> tool windows menu. And I want to add a customized settings for this plugin. I implement ProjectComponent, Configurable, ToolWindowFactory interfaces in my class. So I can create a simple gui for my settings(just one label and textfield) and I can access it in file->settings->project settings. I want to save String in the textfield for my plugin settings but I can't save it. How can I save a string for my plugin settings?