This game is SO hard to get into. I really think I would enjoy it if I could figure out what I'm supposed to do!
Example: I'm starting as William of Normandy in 1080. I have decided that since I have the biggest army and the biggest balls, that I deserve to be King of France. How do I make that happen? The first thing I learn is that if you have an army you can't attack anybody
[) so I'm supposed to disband my army, declare war, then re-assemble my army? I will lose one of my vassal's territory because I do not have succession rights, how do I take it from him? How do I lay claim to friendly territory? The game will allow me to declare war on a neighbour, Count Simon, but how do I know what forces he can gather, or who will come to his aid?
Some tips before you declare a war:
1) Make sure you have a war chest (meaning, that you have a solid buffer of gold in your coffers): depending on the size of the opponent I would recommend at least 200-300 gold. This will allow you to hire mercs should things start going south.
2) Make sure you have maxed out your retinues (if available): your retinues represent your standing, professional army, and are the best troops you got. This opposed to your levies, who indeed you can't have raised before declaring war (I assume this is to prevent a player from positioning his levies prior and then just carpet-sieging the AI).
3) Get allies: marry into families with territory near your opponent, and make sure they hate their guts.
4) Check your opponent's allies: these guys will come to your enemy's aid in a defensive war, UNLESS they're suffering from heavy war exhaustion or are wrapped up in a war already.
5) Bide your time: this is arguably the most important one, you need to wait until they're weak. Wait until they're wrapped up in another war, or until their biggest allies are so they won't come to your enemy's aid.
As for the vassal inheritance warning, that depends on your crown laws. Check
this wiki page. Basically, to be able to revoke his title should at least have your "crown authority" at limited. If you have it at high, the problem is moot completely: titles will not be able to pass from your realm anymore (so this is what you'll want to aim for).
Typically this occurs when he married someone else with a title too (and a different liege), so the heir will have claims to both titles. Depending on who the heir inherits from first (or which title is higher), the title might go to the spouse's liege instead. Murder could be a viable way out of the situation: murder the heir, murder the wife, and let him marry someone else and produce a new heir.
But enjoy the game! Once you start learning the intricacies of medieval politics and court intrigue, you'll have a blast.