1. Use the selection tool to draw around the person.
2. Invert the selection (SHIFT+CTRL+I)
3. Check that the background colour in the toolbox is set to white.
4. Press CTRL+DEL
There are other ways such as copying the selection to a new layer and then adding a solid white layer behind the new layer. This is a better method because you can tweak the original figure with (e.g) the eraser.
PS There are some very good tutorial videos on the web - google "photoshop elements tutorial"