Best advice I can offer to learn from scratch is to just keep playing with it. Worked for me! Then when you 'can' do a few simple things make it your goal to do specific things with the help of a good simple book. I got Photoshop CS4 in simple steps by Louis Benjamin.
Try to avoid internet tutorials as they just seem too complicated and confusing, i.e. no one has vetted/checked the information. Photoshop is a beast and you will be always learning something new!