if you don't understand a thing, starts by reading the manual or the "How to play" guide or even the game rules ... all of them available in-game
ultimately, you can also find help on the forum :
in order to recruit troops :
1) you need to have the necessary technologies to do so (ranged attack 1 for archers, melee attack for spearmen, melee attack and armor for swordsmen, melee attack and armor and war horses for cavalry), as well as
military academy lvl 12) you need the proper building (shooting range for archers, barracks for spearmen and swordsmen, stables for cavalry)
3) you'll need the ressources available for the troops you want to recruit
4) you'll need unassigned villagers to conscript into your armed forces
... when all of this is ready, then you just need to go to menu Hire/Soldiers and enter the number of each type you want/can recruit.
Have fun,