That's not much to go on. Are you trying to make an alpha-channel animation for use in other applications? If so, you should add the comp the the AE Render Queue. In the Output Module settings select a Quicktime Movie in Animation or PNG codecs (they support Alpha Channels), plus RGB+Alpha, Millions of Colors+, and Straight (i.e. NOT Premultiplied).
Media players do not support transparency. If you were to properly render your project to a format that supports alpha channels and you were to play the video on a website or on your computer screen with any of the available media players the background would be black. If you are going to be using a media player then your only choice is to create a solid the color of the chosen background and put that at the bottom of the timeline. If you must have a transparent background for a web page then your choice is to use flash.
If you need to import the spinning globe into a NLE and add it to some existing video then the right thing for a beginner to do would be to add the composition to the Render Cue and then choose Lossless with Alpha from the Output Module presets. It would be a good idea to change the color mode of this template to Straight Alpha.
Give us more detail about what you are going to do with the video and we can point you in the right direction. You owe it to yourself to study up on video formats, video standards, and rendering. That's one area of video production where a novice can get into real trouble and waste a bunch of time.
Thanks I will try that
Actually, if you go to the web site that is under construction you will see what I am doing. where you'll go - Around the world in 80 minutes
I did the globe in AE and masked and deleted the black. I clicked the transparent box and the globe looked perfect in the create window, with the transparency on. I thought It would be a matter of rendering it, but then it turned out to have the dreaded black box. I will recreate it with the suggestions you made and see if that works. thanks. Let me know if when it's on the site it gives you any further information. Thanks