1. You can hold F1 to point to where you want your soldiers to hold. It's a good idea to have your infantry hold in front and have your archers behind them, preferably in a place where the archers are on higher ground.
2. It's a good idea to swear fealty as soon as possible. You get allies and likely a fife, which you get a good amount of money from.
3. Leadership and any other leader skills, such as prisoner management. Avoid party skills, as you can use companions for those.
Jeremus is the best companion. In my opinion, Artimenner is a close second. Unfortunately, they hate each other's guts. Of course, Artimenner is only useful for laying siege to all of Calradia and laughing as entire nations fall to your might before they even get a chance to respond due to how damn quickly you build ladders, while Jeremus is useful for
every fight you will ever have, as he prevents your soldiers from dying with his surgery skill and keeps them ship-shape for the next fight after. Plus, he's basically the Calradia equivalent of Philippus Aureolus Theophrastus Bombastus "Paracelsus" von Hohenheim, and you can't hate a name like that.
So, I would recommend Alayan, Baheshtur, Firentis, Jeremus, Ymira, Bunduk, Deshavi, Klethi for companions. My current save has Deshavi, Artimenner, Ymira, Marnid, Firentis. Any other companions will conflict with these.
See the chart.