depending on the image you could use the magic wand tool which automatically selects edges. i would use the pen tool with 'paths' selected from the top toolbar instead of the default 'shape layers'
once you have it outlined spot on you then need to click on paths (one of teh tabs near the layers area) and right click your path and choose 'make selection' set the feather to 0 and you should have your selection. just cut, copy, paste what ya like from there on. if your wanting the wheel id copy the selection, create a new image and paste. then you can tidy up resize if required,