Do I need to cook the Apple for making puree for 6mnth baby?

Yes you might want to soften the apples first. You can do this by putting the apples in a pan with enough water to cover all of them. Let the apples simmer till they are soft enough to be mashed. http://www.bbcgoodfood.com/recipes/1159647/really-easy-apple-puree

You may wish to steam the apple first to soften it before purée.

Yup, steam for 5 mins. Here's the recipe