When you click to launch HOI3, a window should come up. this is where you can launch the game, get some help and option's, and choose to use the mod's that you may or may not have installed. to play, simply click on "launch HOI3." it also show's your current version, and can tell you if you need to update it.

When you first start up HOI3, you will presented with "start points." these are certain time's and countries in which you can play as to help you to get to grip's with the game. if you wish to pick your own nation and start time, then click the button in the bottom right. otherwise, we shall pick germany.

You should now be in game. as with most paradox game's, when you first jump in it can be quite overwhelming. but don't worry, you will get used to it! we will now go over the UI.
on the top of the screen, you can see your resources. most are self explanatory. supplies supply your troop's, oil fuel's tanks, planes an ship's. money is used to buy thing's from people VIA trade. but, the 3 in the top left are not so. these three are used in your factory's to keep them running. which bring's us onto the 3 number's you can see top centre, this is your IC, or industrial capacity. this is more or less how much your nation can produce thing's to keep your army and you nation goin, such as build troop's, ship's and other thing's like that. we will go more into this at the production section. in the top right, you can see more icon's. from left to right these are:
Manpower, what you use to build and reinforce your army.
diplomatic points, which is used to conduct dimplomacy.
spies, which we will talk about later.
officer ratio, which is the measure of how fully staffed the officer corps is.
dissent, your nation's civil unrest.
national unity, measure of your country's overall morale.
in the bottom right of the screen, you have the map, and a number of different mapmodes, which explain themselves if you mouse over them. usually, people play on the politic mapmode so they can understand the situation better.
(please note that if you have expansion's, then there will be more button's, but because they are of smaller importance, they will not be covered.)

now, of course your army is the core mechanic of the game, and take's a lot of explaining to get the point across, which is why, for the sake of easy understanding, there is a youtube vid below. All credit for the video goes to Michael Broschek.

Diplomacy in this game, if used right, can impact a large part of the war. from making deal's of trade, to inviting other nation's into your faction. it is a crucial part of the game, although it may not seem so. below is another youtube video, again from Michael Broschek, who explain's it much better than i could ever do. all credit for video goes to him.

Production is where you supply and upgrade armies, build new entire armies, navies, and air fleets. without production, you can say bye bye to your dream's of becoming a super power forever. from now on, all videos from Michael Broschek are fully credited to him, so no one moan about copyright :/

Without researching tech's, your nation would be fighting the enemy, who has modern tank's and plane's with ww1 vintage weapons, and your industry would only be able to support half of your nation at best. it is also where you gain diplomatic point's, spy's and officer's.

This is where you see which party is dominating an which are weak, who is a threat, you goverment and those employed, whom all have different benifits and negative's. you can also enact new law's which can help boost certain factor's.

This is where you can send spy's to gather information on other countries, and defend your own country from outdoor spy threat's.

So, now you know the basic's to HOI3, which should get you set of to making a true german empire, or stopping them in their track's. now you can play any nation, set your own goal's and rewrite history! for example. making a strong romania :D