This is a tricky question to answer. The reason its tricky is that when you create a selection, the quality of the selection edge depends on which tool you used to create the selection and it depends on the pixel colors and composition in your image. Since everyone uses different images with different colors and resolutions, the instructions will be different for each situation.
One thing you might want to try is once you've created the selection with either the magic wand tool or the scribble select tool, tap Select > Refine Edge. Choose a small brush size, and brush along the edge of your current selection to refine it. The smaller the brush size the more accurate you can be. It may help to zoom in on the image to really see the edge refinements as you adjust them. For best results, straddle the selection edge with the Refine Edge brush. Here is where you can find the Refine Edge tool:
I hope this was helpful,