This game is not supposed to be easy. At the beginning, you are a fresh recruit sent to war in a time of dramatic crisis, with as much training as you could be given in a few months, which means not so much, and no previous combat experience.
That does not mean the game will be unreasonably difficult; only that you should play as if you really were that recruit. Do not expect to cut through the enemy lines like a hot knife through butter.
If for instance you are not extremely nimble, maybe you should think twice before rushing forward to open up a path for your centurion so he can be the first man over the wall of New Carthage. Of course you can still decide to play the hero, but be prepared to pay the price if something goes wrong. In the same way, you can volunteer to assault the mercenary garrison, but do not expect it to be a cakewalk. They are professional soldiers and you are a green rookie at that point in the game. The most daring choices will (usually) be the most rewarding, but a much higher risk is involved.
That said, during the first battle, if you steer clear of the most dangerous situations, you’ll only face the hastily raised local militia. They are a lot less skilled than you, although you may occasionally meet a more experienced than usual element. Especially in your first combat, make sure to read the tooltips, including the one about the character Stats. Try to keep you Stance as high as possible or you’ll become a lot more vulnerable and do not expose yourself without a good reason. Always look at the indicators under each available action: the more the sword tip is pointing upwards, the higher your chance of success (I’ll replace the sword with a thumb in a future version).
You may not be able to kill even a single enemy in your first battle; don’t get discouraged: it’s normal, you’ll get a lot better with practice. If you are careful enough, you should be able to survive this battle even if you spent your time very poorly during the previous “lull” phase.