just make sure that there are no layers underneather your text layer. You can check the transparency by clicking on the channels viewer button on the bottom of your composition window. It’s the button with three coloured circles overlapping. Select alpha channel. Then you can see which parts of your composition are transparent.
Then if you are happy you just need to make sure you render out with an alpha channel. Use the render queue, and in output module select a format that supports alpha channels (like quicktime animation, or TGA sequence) – then you just need to make sure that RGB+A is selected under the channels option.
Also, if you are importing into something like FCP I normally choose straight alpha channel instead of premultiplied.
Hope that helps, let us know if you have any more questions