If you truly have a low testosterone condition, it's going to be very hard to lose fat; in-fact, it could be impossible unless you basically starve yourself.
Some people like to say you should be lean before you ever start supplementing with steroids; it is true, you shouldn't be obese. If you're obese, the blood pressure and cholesterol issues are more than likely be too hard to handle, but even then, if you're suffering from low testosterone, some form of TRT therapy at a low dose might be warranted.
If you're not obese but still carrying a little fat, there's nothing wrong with starting now...in-fact, in my opinion it doesn't make sense to wait. If you're going to use steroids but lose some fat first, well, this means you're going to lose muscle mass too and that makes no sense.
As for correcting a low testosterone condition, you have to supplement with testosterone...it's the only way you can remedy the problem.