You can easily use the ingame editor to build maps using the existing units, add simple victory conditions, and get your battles going!
We suggest using a fixed, unique prefix when naming your campaigns (a good example would be your forum username) - this means there won't be any confusion when you share your campaigns with other players. For example, if your forum name is Patton, then you would begin the names of all your folders with Patton_, and also the display name for your campaign.
Once you have created a campaign, you can create missions and save them into the campaign, all within the game, and all without needing to edit a single file!
The editor provides powerful tools to placing objects, units, and terrain. The editor has tutorial popups to give you initial instructions on how to begin, and more details about using the editor are included in the game manual. There is a shortcut on the Battlefield Academy programs shortcut, or you can download it from here:
The first place to start is by taking a look at the Map Editor Guide. This will clue you in on the basics of creating and editing maps.
http://www.slitherine.com/files/bbc_ba/modding/BA_Map_Editor_Guide.pdf
Remember that the Scenario Design Forum is the best place to ask any questions you may have.