A gentle fade as opposed to a wicked slice can be a great shot to have. But if you're hitting it when you don't want to, it's because the clubface is slightly open (pointing right) at impact. Your swing path is probably fine, so to lose the curve and hit straighter shots, you need to square the face at impact.
The Drill: Take your address with a 7-iron, then place a headcover directly opposite the big toe of your left foot.
Make a swing and try to hit the headcover with the toe of the club. Doing this requires you to rotate your forearms so the clubface squares up at impact, then closesi.e., points leftas you swing into your follow-through.
Repeat the drill five times, then tee a ball and hit it while thinking about striking the headcover with the toe. I bet you'll see the ball start straight--and keep going that way.