Filter > "Extract" to get to the extract tool.
Use the highlight tool to draw around the outside of the image, your aim being to have about half the object and half the background highlighted. Then, once you've gone all the way around, use the fill tool to fill the area you want to fill. Check the preview then accept your changes. Don't forget to do it on a duplicate layer first to avoid ruining the first if you mess it up. If it's too rough, go around the edges with the eraser.
Alternatively, if it's a simple image, try the magic wand as people have mentioned and mess with the tolerance levels along the top.