I'm not a big warmonger, so let's see what I can do...
1. what to reasearch early and later on
Having an advanced military is obviously important, so going the lower branch of the tree (Bronze Working, Iron Working, etc...) is probably going to be your main focus. A very important tech is Mathematics, as it gives you access to the Catapult, which is pretty much necessary to capture any city of a considerable size (except maybe if you're The Huns, in which case the Battering Ram is freaking awesome).
Having said that, do not undervalue important techs such as Writing and Education - as I said, having an advanced military is really important, and you need these techs to keep ahead of the rest. (if you are playing as Assyria, you can steal techs by capturing cities, so you have bigger wiggle room to ignore the top branch of the tree - those will be given to you by warring to a certain extent)
Because of this, you might want to keep your wars short and focused in the early game, so you can concentrate a bit on your main city's infrastructure.
2. how many units and what kind ( not related to a spacific civ ) should i produce before going to first war and after
To capture cities, Siege Units are a necessity - I've found that two Catapults is usually not enough, three is good and four is great - more than that is overkill, probably... It's totally possible to win a city with only two, certainly, but you are at risk of losing them quickly. Have a good amount of Melee units, to protect your Catapults during Siege - you can go all six to surround the city, but it might not be necessary. Also, if possible, have a few Mounted and Ranged units to take care of any wandering enemy unit that might disrupt your siege...
Do not forget to keep some units at home, to defend it (Ranged and Melee), and have some reinforcements around - you WILL lose units along the way...
Late game, Air units! Have as many as you can, each one of them will be useful. They tend to get really low on HP, so you can cycle between them, leaving some to repair while other go bombing, and vice-versa...
Go to war when you feel you have enough units to attack the civilization you're aiming... It's really all I can say - if you feel the amount of units is not enough, then build more before declaring war. This will depend on your skill and knowledge of the land and enemy forces - use your Demographics screen to check things regularly.
Also, note that some Civilizations are harder to take than others: The Shoshone have a bonus when fighting inside their territory, Ethiopia has a bonus when fighting bigger civilizations than themselves, Morocco has the Kasbah that provides bonus to defense, etc... Study your target well, and exploit their weak points!
3. should i attack city states or just civs ?
City-States are really useful if you're allied to them!
Usually more so than belonging to your civilization - they provide you Happiness (and you will need as much Happiness as you can get), units, etc...
Not only that, other players get really angry at you if you destroy City-States, and you don't want to be declared war by the rest of the world, so be careful when doing it.
Obviously if a certain City-State is in a prime location that would benefit your empire, then go for it, but beware of the consequences. Declaring war and destroying too many City-States will make all of them forever at war against you!
Also, they might make good targets early on to promote your units!
Generally though, you will want to concentrate on major civilizations - they are your biggest threats!
And I guess that is all I can say!
I'm not big at war, so there might be some huge mistakes I'm doing, so pay close attention to some better players out there too!
Maybe watch some videos on YouTube to get the feeling for things...
