Hey, you're giving French teachers of English a bad name
OK, I'll try to help here - though the French-speaking forums of PC4War would also be a good place for you to ask this kind of questions.
The AI level only changes how many points the AI gets to spend on pretender design, not how well they play; so if you have trouble with the game's mechanisms, it's likely that any level of AI will give you trouble.
Pretender and castle choices vary greatly on what your strategy will be. For a beginner, I suggest you don't try to make too heavy use of your pretender, so a good researcher, possibly with many paths of magic for site-searching, would be a good idea. Castle-wise, the 80 pt castle with 40% Admin is a good standard choice: the Admin bonus is important, as it commands how many resources you will "drain" from the adjacent provinces into the one with the castle - since you can build your national troops only in provinces with castles, this is important.
For early games, I suggest playing a nation with not too many Magic paths. Ulm (basic theme) is pretty straightforward: only one mage, specializing in Earth magic with a little Fire, and two special abilities: forging items (research Construction for that) costs them 25% fewer gems, and they don't suffer a Research penalty in Drain dominions (this lets you take Drain3 as your dominion, which gives you 120 "free" points).
Since Ulm needs huge amounts of resources for their heavily armored units (both the various types of Infantry, and the cavalry), you really must take a good Productivity bonus as your scale (+3). Taking Order never hurts (will mostly increase your Gold income), even though Gold isn't the most important thing for recruiting Ulm infantry (Cavalry is a different matter).
As for your troop placement... well, your Infantry is slow-moving, but tough: you typically will want to place it on the front of the battlefield (right side). Small squads let you separate them better, but larger squads have better morale, and since you don't have easy access to level 3 priests (which is required for Sermon of Courage, the basic morale-enhancing spell), using largish squads (15+) will be often better. Anyway, it all depends on what you will be facing; experiment for this, and scout neighbouring provinces to know what kinds of troops, and how many, are there.
On the army screen, you have a square for each squad and for each leader, with a smaller white square for their positions on the battlefield (and red squares for the other troops). Click on it to place your troops somewhere else, remembering that left is back (away from the enemy), and right if front (closer to the enemy). Top/bottom are left/right flank. Then you can give orders to the various leaders and troops. When giving spell orders, keep in mind that spells often have a limited range (when watching a battle replay, press "g" to see the battle grid: range is counted in number of squares).
And, try to read the tutorial (linked on the Illwinter site, among others). It's about playing Jotunheim, which may not be the easiest nation to start with, but it's very detailed and helpful.