Quick summary of tips:
(Assumption: not playing Monarch or above)
1) Strike someone early
2) Be friendly to those you don't plan on conquering right off.
3) Don't leave your cities open to attack. Civ's you thought were friends can be tempted by undefended cities.
4) If you're behind in tech demand it from a weaker Civ. If they don't off to war you found your next target! Let them sue for peace, but it will cost them tech..
5) You were beaten to a Wonder by a Civ that's not an ally. Bingo ! found your next target.
6) I use Spearmen (defender type unit) to go along with my swordsmen, archers (offense type unit). I realize units will change in other eras but send your pike muskets along to defend your cav. This serves as defense for your attack units and also to put a defender or three in your newly claimed town.
7) I'll quote this one, "Never, never, never ever assault an enemy capital on a hill across a river. " Well, each part adds to the defense in the combat calculation ( Capital city, on a hill, crossing a river ) Only do that if you realize you can suffer heavy losses.
8) If you care about Reputation dont abuse treaties. Who cares about the enemies right? But this influences allies also.
9) 'Combat Engineers' - divert some workers to build roads to connect your cities. Get troops quickly, plus the city with unrest gets benefits of your empire's luxuries.
10) After taking a city may want to leave all those units inside it a turn to quell unresters.
11) Arty is your friend.

Personally, I dont use catapults, but from cannon on up I not only use to to defend cities with but use them to soften up the enemy. Usually defend them with one or two defense type units.
12) Gov'ts - Despotism is first but its not great. Personal preference choose between Monarchy or Republic. May come down to play style which is best suited for you. Republic if your wars are fairly short. War Weariness can start putting a damper on the war effort. Monarchy does not have that problem but the workers bonuses arent as much. If your wars are real short in duration its possible to have wars in democracy and not need to switch. I've not had good experience with Communism, so I won't comment on it. But a long dragged out war can erode enemies from an efficient gov't republic/democracy into it. Military/Religious may be a good civ choice if you like being able to switch gov'ts quickly.
Hope that helps get you down the path of warmongering.
Check out the War Academy section of Civfanatics, also. It's a good repository of strategy articles.