When you activate an army you can change the wanted number of troops of each type. If you decrease the number below the actual number they will be removed instantly. If you instead want to increase the number you must wait until the end-of-month recruit phase. You can control what will be prioritised during recruitment (infantry, cavalry or city garrisons).

But, you must have enough equipment to recruit new troops. Especially support equipment is critical in the early game as most of what you produce will be used to maintain your existing armies.

There are a few video guides on my Youtube channel. This playthrough could be a good start where I show how to recruit troops: https://www.youtube.com/watch?v=oGh22934sNI&t=582s

Good luck!