You don't loosen "some nut." See below - the fixing bolt circled at the bottom is what you loosen so that you can pull the cable through it to tighten the adjustment. The barrel adjuster, circled above, is generally for fine adjustment. The barrel adjuster also has a quick release on it that can be helpful when you need to remove the front wheel. The caliper also needs to be centered (make sure the wheel is centered in the frame and the wheel is straight/true).

Google adjust sidepull brake and look first at the sheldonbrown or parktool results for best guidance. Both sites have almost every procedure you would need to perform. It's best to follow good written instructions and use videos to see the procedure live.