I have download a tiff file with elevation from http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp and I try to export in file text or Excel the coordinates and the elevation and I don't know how to export this data in text file.
Using QGIS select Raster/Conversion/Translate (Convert format) ... from the menu. Select the ASCII Gridded XYZ file type and set the output file. It will execute a gdal_translate command as you can see it in the dialog. gdal_translate -of XYZ input.tif output.xyz
Note: it will use the raster value of the first band as Z value if you have a multiband image (e.g. RGB).