When you're creating the text in Photoshop, you need to select and transparent background instead of white which is the default one. To do this, open Photoshop, press Ctrl + N key combination, enter your file dimensions and from the Background Contents list select Transparent. After that, create the Text and save it as GIF, PNG, BMP or TIFF because only these formats support transparency. Personally, I recommend you save the file as PNG.
Also, using Photoshop, you can remove the white background from your already created file. If you have the PSD version of the file, import it in Photoshop and double-click the Background from the Layers panel to transform it in layer. After that, select it and set the Opacity to 0% or press the Delete key to completely remove it.