There's been mixed opinion here about the use of Windex. I'd recommend against it - the ammonia.
Generally, a very soft non-linting cloth (no lens tissue!) and just plain water should be able to remove the vast majority of the gunk that's removable.
If you absolutely need a more active solvent, if I remember properly, NIST recommends as a sort of last line of defense to be an alcohol based cleaner.
And, of course, wipe across the data track, not along it.
In a pinch, I've often used (a) a relatively clean cotton or cotton/poly shirt, and, as needed, (b) a bit of Universal Museum Solvent.
Leo Kerr
P.S. What do you mean, you don't know where to get UMS?! From a relatively clean mouth, spit on your hand, lick your finger, whatever, and use it!
For light scratches you should use Turtle Wax. Yep, Turtle Wax. I've seen it used in professional recording studios to "fix" CDs. Use just a little. Buff it on and buff it off (don't rush).
Simple Green. Highly diluted Simple Green (read the bottle for the window washing dilution, use it) works great and it's ammonia free. Also, use a good microfiber cloth like the type used to clean computer monitors and television screens.