Alakazam, she just wants to know how to win a war! ...so, o.O!
Ok, first, get a lot of these :
(or trebuchets, cannons, depending on what you can get)
Now, your government is Monarchy, right? It better be!
March up to a city with the artillery, along with some defensive units and some attacking units, all in one stack. Bombard them until you run out of artillery, and then attack with swordsmen, horsemen, knights, or whatever. Capture the city and quell the resistance by putting a lot of units in, and starve the population by changing all the citizens into tax collectors. The next turn, get those artillery units on their way to the next city, no stops. Keep them guarded with spears, musketmen, whatever at all times, and always bring enough offensive units to take the next city. Starve each city you capture down to one citizen, until you've either conquered everything or you're signing a peace treaty (cause they got these island cities that too much of a pain 'nstuff)
For every attacking and defending unit in a stack, have around 5 artillery. You're average enemy city will have 2-3 defenders, plus the odd archer or piece of artillery.
POST A SAVE!
edit: Just occured to me that you might be female.