You need to be more descriptive as to what the issue is. Are you getting a white model when exported? If so, make sure the texture is linked as per
here. If you're refering to inability to view the texture on the model while in Blender, also see
here.
And a few simple basics: If you saved the file as any other type, ensure that you resaved it as TGA (though I'd personally recommend you use PNG instead; be sure to turn off RLE compression if using TGA), not just tried to rename the file with a different extension. If the G3D viewer throws any kind of error, post it. If you've renamed the file ever, be sure to relink it in Blender (as per the links above). If the file is in a format besides TGA or PNG, convert it, Glest and its forks don't support, for example, PSD (Photoshop) or XCF (the GIMP) won't work. Yes, BMP and JPG are supported, but there's no particular reason to use either, as BMP is just TGA without an alpha channel and JPG has lossely compression.
None of those work? Either be more descriptive or post your model (and texture) here. Upload them in an archive to a file host such as
Mediafire and post the link.