Okay:
Go to your local grocery/convenience store and buy a 2-pack of Mr.Clean Magic Erasers (Something like 2.99 per pack). Turn OFF your MacBook and (this is not entirely necessary) just for good measure, remove the battery. Take a dry soft cloth (a glasses cleaner is perfect) and wipe down the MacBook to remove any dust. Take one of the magic erasers and soak it, but wring it out pretty good. Start working on your MB. You might have to work at one certain smudge more than once but they usually all come off. You can clean the Keys and the trackpad, too. If you think you might have gotten some water inside the laptop, just let it dry for a couple hours after you're done (meaning, don't turn it on). Once youre done, don't throw the Magic Eraser away. Just dry it out, you can use it 3 or 4 times, just like a sponge.
Hope I helped.