Macbook lagging

Help, my Macbook is behaving very slowly. When I bought it, I was very impressed with its speed, but since, it has been sloing down rapidly, and in recent weeks its starts lagging for even the most simplest tasks. The spinning thing comes out for a few seconds just to select a photo in the Finder although the only other application open is Google Chrome. I have heard about stuck processes, but I cant find anything that takes up substantial cpu percentage. Please help me? The fan speed can go up to 6000rpm too, just to add.