The quickest way I have found aside from selling resouces is to click on cities (right side of the screen), go to the column where it shows how much money is brought in and at the top is a "dollar bill" symbol. Click on that. It should then show what province brings in the most to least. Build Arms Industry buildings in those provinces, from most to least. That should start building up yer income. For economic reasons you should already be building Arms Industries to begin with. That will be the only way you can manage to wage war with strong neighbors. If not, yer gonna lose regardless.