I assume you mean a campaign map, as all you need for battlemaps is included in the game's own map editor.

Here's the best place to start: https://forums.totalwar.org/vb/showthread.php?t=13155

The tools you will need include:

BIFReader
Mithel ImageConverter
LMM
A good graphics package (eg Photoshop - you need to be able to operate in both indexed and RGB modes, handle colour palettes, draw things pixel-by-pixel in some cases, define colours precisely etc etc)

... and an endless supply of patience

Map making will tie into virtually every part of the game, so new maps need their own startpos files, probably their own unit_prod file, new region and faction_specific files and heroes.txt file, new names.txt file, minimap and mini_lukup and front-end lukup maps too. A LOT of different htings need to be right for a new map to work, and when something is wrong, you have very little indication of the error - only a CTD. Debugging is a nightmare, due to the complexity and lack of information.

Seriously, put aside about two months where you expect to do nothing else with your spare time. Once you get over that hump, and get used to the tools and have everything set up and running, you should be able to spit out new campaign maps.

Good luck, and if you have any specific problems, just holler