While building up your economic infrastructure, move your starting army to the lower left corner of the map. Be sure to constantly use dwarven riches every time it recharges to keep that income flowing. Keep building mines until you have about 12. Once your first mine is done, use the Dwarven Riches power (if you don't have it, purchase it immediately). Build two extra workers (four total is a good number for when you're starting out, but you won't need them all later on) and start filling the map with mines.
Erebor is a very wide and open map, so take advantage of that. Never build a mine lower than 95% efficiancy. Make sure you get every last one, then start building replacements. All your starting mines have efficiancy somewhere around 50%, which, as you start to add more mines, will actually lower your income more than it will increase it. Your first course of action should be to delete every mine in Erebor.
