It, of course, depends.
You want to change that which affects the photograph negatively the least.
If you are shooting scenery, you have a decent latitude in terms of aperture. You ideally want to be above f/8 and can easily go as far as f/22 (although you may lose sharpness after f/16). You have even greater latitude in terms of shutter speed, anywhere from 1/15 with a tripod (or less!) to 1/5000 (although if it's a sunny day you'll probably be closer to the faster end of the spectrum).
ISO can vary anywhere from ISO100 to 800 (or even higher with newer cameras). You will notice noise less in brighter scenes than darker.
Before you can decide what to change, you need to decide what is important. I am assuming you are not using a flash, because that will dictate your choices for you to a greater degree. So what is important?
Are you trying to freeze something moving, such as a bird, or are you trying to give water that flowing texture? If so, you will want to fix your shutter speed at something appropriate, 1/500 for moving animals (or faster) and 1/15 or less for water. If it's just pastures and mountains, then it really doesn't matter since nothing around you is moving.
Are you trying to capture something up close and personal, or do you want sharpness throughout the frame? If it's a macro of a flower, you will want to increase your aperture as much as possible, ideally hitting f/1.8 or 2.8 depending on your lens. If it's a scene you're after, then get up to f/11 or higher. Use the depth indicator if you have one.
Once you have those dialed in, you mess around with the other two within the constraints of your scene.
Is it dark, moody, and moonlit? You will want to set your ISO to around 400, and see where your shutter speed ends up. Do you not have a tripod? Then set the shutter speed to the inverse of your focal range. A 50mm lens should be fine hand-held at 1/50 s. Now adjust your ISO again until your exposure is right. Or, have fun, and set it until it's wrong :)