You can apply local adjustments in two different ways. One is the Adjustment Brush, which lets you paint a mask over the image to select where you want to make changes. You have full control over the shape of the brush and the Auto Mask feature makes selecting just the right areas very easy. The other way is to use graduated filters which create a gradient mask over the image, which makes it simple to select large areas.

To try these out I used a photo of a leaf floating in the water I took in Deep Cove.

Before: These is the image using the default settings, pretty flat.

After: I used two graduated filters to darken the top and bottom of the image and a mask over the leaf to increase saturation and sharpnesss. I also cropped the image a bit to move the leaf toward the bottom right corner.

These changes make the eye wander a bit more towards the leaf, which now pops out a lot more. And the graduated filters helped get rid of some of the flatness of the original image.