Hello

I'm interested in creating a custom zombie map.
I've never used modtools or such things so where can I inform myself about making a custom map and from where to start?
What do I need...?

Am planning to re-make a MW2 map, so I was wondering, is it possible to convert the whole map or I have to re-make it from scratch? How to use textures/models from MW2 and then add them to WaW?

You have to make it from scratch

You can retrieve models and textures from MW2 by using Lime, a tool would normally find at http://www.tom-crowley.co.uk/downloads but currently at http://www.tom-crowley.co.uk/temp (download "Lime - 0.9.9.96").

As for the basics of making a map, here's a playlist of tutorials I always link to newbies because I don't know any others https://www.youtube.com/watch?v=TxDSly78HfA&list=PLVcaZ3TuoHV8gsSgGolFmrNT-KKE5ldN8&index=1

As for MW2 maps, you will have to remake the map from scratch. It's not currently possible to decompile CoD maps (other than CoD 2 maps), but as I said earlier you can get all the models and textures, so a remake is possible but takes work.

Here Is another tutorial series that you could follow: http://www.youtube.com/playlist?list=PLwlA8xZdjWAg0o9EGpticKPRm8LYZ6yk2