First let me say there are some great lets play out there, Arumba has some that are more designed for beginners, that helped me when I was new.
Techs - first I have to ask, do you have The Old Gods installed and running? Without it I believe it uses the old system, which I don't remember how that works.
Casus belli- There are a lot of things to this. Generally against other religions you get a holy war CB, which is good for all of a duchy. For the most part, against the same religion you will need a claim or to press one of your vassals claims. You can get claims through marriage or fabricating. All children of a ruler get a claim, and claims last 3 generations. So if you are king of England, marrying a princess of France, your children will get a claim on the kingdom of France. There are tons of different kinds of claims and it is very in depth so I would suggest looking at the wiki article for it, to see it all.
http://www.ckiiwiki.com/Casus_Belli
Too many duchies - If you are a duke you can have as many as you want if you are independent. If you are under a king other dukes will be jealous and plot to weaken you. If you are a king or above, you are allowed 2 ducal titles, any more causes negative modifiers from your vassals, -10 for each above 2. Why? Because they want them, and its not fair that the king is hoarding them.
Army composition - for levies you have very little choice, only what buildings you build can effect this, and then not very much. For Retinues, well that is very complex and depends on what culture you are, there are various thread about that, and they can explain it better then I.
Education - for beginners, ALWAYS educate your sons. Letting someone else educate your children and they will end up with bad traits. When they reach the age of 15, I recommend for newer players to have a grey eminence educate them, it gives +9 to diplomacy which goes a long way to making your vassals like you.
Council - generally you just want the best stat person for each spot on your council, but you may want to pay attention to your spymaster, and make sure he/she likes you. If your spymaster joins a plot to kill you, it is a huge jump in plot power and could very well lead to your death.
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