Invading Other Countries
First, you build a city nearby an enemy country you think you can defeat. Have workers completely surround the city with fortresses and then put 2 of your best defensive units (pikeman, spearman, musketman etc.) in each fortress, and 3 in the city. Then you declare war. They will waste all their offense trying to take the city. Unless you are way behind in technology, they won't even be able to wound your guys because of the fortresses. While defending it, have every other city keep building you best offensive unit (like cavalry, tank, modern armor etc.). I suggest you should make about 15. After their offense is spent, invade their cities like crazy without having to worry about offensive units intercepting you.
More Commerce
Build roads in every square inside the civilization's boundaries. If a citizen works on a square with a road, they produce more gold, causing more production of commerce. Building so many roads also helps units get around faster.
More Food
If you find a river next to or within city boundaries, you should irrigate the squares producing no shields in the city. This could keep citizens from starving to death. Also, you should build the Pyramids to put a Granary in all cities, so more food is stored.