Before I go any further, I must say... these are some of the most noobish (but not stupid, there is no such thing as a stupid question in my opinion) questions I have ever seen.
First of, let's explain how you declare war.:
In order to declare a war, you need a Casus Beli also know as CB, frabricating a claim is a way of aquiring a CB.
Secondly, the step is rather simple, you go to the diplomacy view of the character you wish declare war on simply press the button that reads "Declare war" and game will show you a screen which shows what happens if you win, if you white peace out of the war (think of white peacing as a draw), or what happens when you surrender.
Yellow colors on the diplomacy map view are people you currently have CB against.
(Rememder, this is for CK2)