What you want is referred to as a transparent image. The easiest way
to do this in Adobe Photoshop CS (version 8) is as follows :
1) Open up Adobe CS as normal. Once it is open, click on File > New.
2) In the pop up box, put whatever size you are going to make the image. Next,
you will want change the Background Contents to 'Transparent'. It's down
near the bottom of the choices in the select box.
3) Now you have the beginnings of your tranparent image. You can type your text
in over the transparent background.
4) Once you have your text as desired, select File > Save For Web.
5) In the pop up box, make sure you are saving as a GIF type image. Select
the transparency box if it is not already selected.
If you are planning to use this image on a colored or textured
background, you'll also want to choose a 'matte'. This gives the text
(or image) seemless edges to blend with the background.
An example :
I have a website with a background color of #E1E1E1, which is a light
grey. I would select matte, and then enter #E1E1E1 in the pop up box.
To assist with this answer I used my first hand knowledge of Adobe CS.
If you are interested, you can read more on transparency (and
antialiasing) at the following website :
AntiAliasing and Transparency
If the text loge you are making is very light colored, you may have a
hard time seeing it on the transparent background. The work around for
this is to simply flood fill the background of the transparent image
to make your text visible while working with it. Once you are ready to
save the image, you can simply delete the background layer.
Please let me know if you need any clarification, I'll do all I can to assist.