You'd need to edit the mission SQM for map changes and what not, pull it from your server box and use the editor on it to make your changes
There's lots of places to download mods but just because you can download them doesn't mean you'll have the permission to use them on a public server (So it's best to download them directly from the mod creator, or ask the creator if you've obtained them through a 3rd party)
No need to be sorry, everyone has to start somewhere
I recommend joining the discord if you haven't already - https://discordapp.com/invite/FzgeVxw