Army composition:
Depends on the country you're playing. You want stacks that can work well together when grouped up, some people like stacks that can be split evenly in to 2 for easier occupation. Personally I go with 4-5 inf, 2 art, 1 eng, 2 dragoon or hussar or a mix. Hussar better for occupation dragoon for combat. I'll usually bring stacks to 10-11 units total for much of the game. I find this works quite well, each army is nicely rounded out and when grouped up they pack quite a wallop. I don't typically split because I don't like doubling up on the engineer. Late game I'll probably swap out the cavalry for planes and add a couple armoured.
Tech:
The very first tech you want unlocked is medicine. Population is KING in this game and medicine has 3 + pop growth modifiers you want ASAP! After that you really always want the + research point (RP) techs as soon as they are available; education techs are also highly important especially the 4th one as it unlocks Darwinism a +50% education bonus. If population is king, literacy is queen.
Engine techs will be very important early to help squeeze capital out of your nation, as well as banking techs for tax efficiency; at the start even at 100% tax it's really only like 28% due to low efficiency. As tax efficiency goes up and engine and other techs help get your RGO's (local goods production basically) producing more you can lower the overall tax rate.
Army organization tech is one of the most important combat techs, keep it up to date, also small arms especially machine guns and bolt action rifles.
Economy:
The most complex and frustrating part of the game. Early on you can largely ignore mass industrialization with most countries, a few factories is probably fine. Get those RGO techs early to get the whole country producing more of nearly everything. As mentioned tax efficiency is super important.
When it comes to factories, by 1860 or so you can really start working on them. Factory output techs are very much needed to get things running best, as well as input reduction. Input reduction will mean your factories will need less resources to produce, output helps with making more and throughput helps with overall efficiency.
Liquor tends to be a very profitable and stable industry, as does glass and to come extent wine. I usually have a liquor factory in most every state to keep some stable employment everywhere.
Railroads are also your friend, they increase both factory and RGO efficiency/output, help with army movement and increase the supply cap.
Armies and navies are very expensive to be wise about how large you grow them and when. At the very beginning most of those Man o' War's that so many countries start with, are kinda crap and VERY expensive. I usually disband most nations starting navies. I don't really worry about a navy until I have Steam Transports and Commerce Raiders + Ironclads. Keep spending for the army/navy low when not at war to save money, just remember this affects their ORG, so they'll need time to recover. If you're planning on declaring war raise the spending to full a couple weeks in advance.
Resources:
industrialization at it's most basic takes iron, coal and some sulfur. Many of the goods that RGO's produce will be needed in some factory, every factory will tell you what it needs to make what it makes. Having a factory in a state that produces the required inputs gives a very small bonus. Otherwise it is just good if what you need is produced in your nation. There are no supply routes in this game, blockading is more like "blockading", it gives warscore and hurts the local economy but does not actually stop the flow of goods. Goods in this game magically travel, seriously. From wherever they are to wherever they are needed, any country can sell/buy from any country
All goods produced go to a few different places. Supposedly your nation gets first grab at anything you produce, either RGO or factory. From there it goes to the world market. Every day the nation in the #1 spot gets first grab at the world market for their needs, then #2 and so on. If you're stuck in 30th place you may never see what you need if supply is short. Supply/demand can be seen on the trade page. Early game this is more of a problem, but with RGO and efficiency techs as the game goes on this gets better. Being in a sphere messes with the buy order a bit and some of your produced resources will go to the sphere master
The general rule in this game is that you really want to have as much of the raw resource production as possible under your own thumb, either directly owned or sphered. Having a nation as a puppet gives you no special access to resources.
Umm...I dunno ask questions I guess. This is my fave game I'll answer what I can. Especially for the Emu.