I have an idea (don't we all) for making a new map from a real WWII battle similar to as Market Garden using the same models, different skins but also different opposing flags.
Can I do this by creating a new level or does this have to be a MOD? When can you make just a new level and when does it have to be a MOD?
this is off topic but what software do i need to make a map
Go to this site. There is a list of tools. You may want to download the Unofficial SDK from this list.