There are three files op2 needs to run a mission:
1. .map file (graphical map file)
2. .dll file (mission file)
3. .txt file (tech file)
1 - As Sl0vi said, the mapper only does the map file (*.map) that holds the locations of graphic tiles and a cell type (it tells the game if you can move over the tile and how fast you can do basicaly). You can also make your mission use other maps already created.
2 - The .dll mission file is the most complex. The file needs to be coded in C++ with all mission details, and computer player AI. However (as Sl0vi already posted) Eddy has created a program to help with this task. Once your .dll is compiled, you need to name it with a mission prefix like this one: "ml4_" which tells op2 this mission is a multiplayer LoS for 4 players. Op2 needs this prefix to open the files.
3 - The .txt file is the easiest file to get. Usualy you won't even need to make one. However, techs define what a player can build and research. It also tells the game things like how much hp a unit can have, how far it can shoot, how much it costs, ect. The tech multitech.txt is used by most games, and is the standard Op2 tech.
Finaly, always look in the wiki for your answers, or other old topics. It saves everyone time.