Are you making a Warcraft 3 custom map or a Starcraft 2 custom map?
For either editor, you need to start off small. I made the worst and lamest maps when I first started (and I still am). With each map I made, I learn't something new. There is lots that goes into a map. Terrain, unit data, triggers, game design, balance, creativity, etc.
Where to Start:
- Start by simply opening the editor and looking inside. For your first map aim really small. Just put a couple of units on the map for each player and have fun moving them around
- Next I suggest you edit the unit's damage, armor and hit points. Test the map and have fun killing units when you have 10000 damage.
- Next try making a unit pop up on the map every 5 seconds. Test the map and camp the spawn zone with your 10000 damage mega unit and kill them.
You aren't really trying to make a proper full fledged game at that stage. Have fun learning new things in the editor, then tackle making a game. Remember your first few games will probably be crap but practice makes perfect.