Firstly, make sure you have created a scene and added a source to actually be seen. After that, you'll need to turn on 'Preview Stream' or be streaming in order to see your stream
You need to be previewing your stream, or actually streaming and click 'Edit Scene' to move things around. You can also hold Ctrl while moving a source to prevent it from snapping to the screen edges, Shift while resizing to ignore the aspect ratio, and Alt while dragging the edge of a source to crop.
Are you trying to stream a fullscreen game? You'll need to hook into it using the 'Game Capture' source in OBS. As well, OpenGL games like Minecraft have issues with Window Capture and you'll need to use Monitor Capture to show them. If desktop capture is showing up as black, it indicates that you either need to update windows or update DirectX.
Windows XP unfortunately lacks DirectX 10 features and the audio subsystem present in Vista and up that OBS makes use of, and as such, will never be supported. Mac and Linux are planned in the future.
Make sure you have the latest version of DirectX and that your video card drivers are up to date.
Your capture device may still be unsupported -- please visit the Problems and Bugs forum and read the thread regarding red output from devices.
Check out our settings estimator for general recommendations based on your setup