I would like to make dents in an item I have made in blender(a old key). What would be an easy way to get dents on the item?

In the end you most probably will want to do them with textures (normal and/or displacement maps). Creating the textures may be as trivial as painting some dents in your favorite image editor (i.e. black-and-white displacement map), or less trivial: increasing your polycount considerably in order to be able to model them (e.g. in sculpt mode) so you then would be able to bake normal/displacement maps.

Stan is right. Easiest way is to just go into an image editor and make some random black marks on a white background. Then pull it into blender and use it for normal mapping. I never like to displace if it can be avoided. Adding crap ton of verts just to get a scratch layer is silly.

thanks for the help Stan Pancakes and brent newton.