There are lots of tools for this:
1. The polyganol lasso tool on the flyout under the lasso tool icon. Zoom in and click carefully around the border. Hold space bar down to drag around the image. Use backspace to remove anchors that you've made. click the add and subtract icons in the top toolbar to add and subtract sections from your selection.
2. Quick mask tool. You'll find the button for this just under the foreground and background colour squares in the tool pallette. Switch to quick mask mode and paint on your image in white or black to select an area, and then click out of quick mask mode and you'll see your selection
3. Extract Filter. Filters>Extract. Play around with it, you'll see how it works.
4. The magic wand tool can be useful if the background that you are extracting from has a fairly flat colour.
I'm sure there are others that I've forgotten. I'd suggest the polygonal lasso tool for most things (my favourite anyways).