Water and a microfiber cleaning cloth is all you really need. Use a new cloth dedicated to the computer so that it's not contaminated with other cleaning chemicals. Apple used to include cloths with some LCD screens that had a slightly different texture that works particularly well, but any microfiber cleaning cloth should be okay. (Also good for cleaning eyeglasses.)

What I do is dampen a small part of the cloth with water and wipe the screen. Then I go over it with a dry part of the cloth. This won't do much with a standard cotton cloth or such, but with the microfiber it works well. If it's been a while and the marks are really stubborn (generally oil from your fingers, either from touching it or from the keyboard while it's closed) I might have to repeat the process to get the best results. But it always has given me results. There are pads you can buy to put between the keyboard and screen while the laptop is closed to protect it somewhat. I keep meaning to try one but after a few years now of seeing them I still haven't.

Do not use a paper towel, tissue, etc. Too scratchy, even if it seems otherwise. (A little tiny dab here and there probably won't hurt, but don't take chances when trying to do a good cleaning.)