Because you are new to the editor, a Footmen Frenzy map might be a bit...tough.
What you should do, is have the same layout as the footmen frenzy you are making (like an X with a shop in the middle or whatever), but have the units be manually bought (the automatic spawning is somewhat confusing). And for the heroes, start out with melee heroes. Make custom units if you are feeling confident but if you aren't, use melee units as well.
After you get your first playable version, most likely you will have learned much from the editor compared to the start. Then, you should make custom units and custom heroes and adding more levels to abilities (so they can go up to level 10 or whatever you wish).
After that, you will have custom heroes + units = a footmen frenzy map except for the automatic spawning. You should then ask on the forum if you need help for the automatic spawning trigger.
If you attempt all the custom heroes and units and the spawning triggers and other more difficult triggers, then you will feel overwhelmed and may not even finish a playable version of the map because of discouragement.
-Demonfaze