There are no "requirements." All that is minimally necessary is that you walk on to the property of a monastery, find the abbot, and ask him for his blessing to live there with the intention of becoming a novice. If he gives you his blessing, then it's off to the fields, to the toilets, to the kitchen, etc. for lots and lots of hard work and manual labor. The younger you are, the more hard labor you'll get, to wear down your body, decrease physical temptations, teach you humility, and test your mental and spiritual resolve. Do you really want to be a monk, to die for Christ every day, to experience the martyrdom of asceticism? Or have you read some books and think it would be great to dress like a saint?
When Elder Cleopa left his home as a young teen (14 or 15, I think) and presented himself to the abbot at Sihastria, the abbot gave him a stick and told him to hit a big tree that was outside the monastery walls until he had chopped it down, saying the Jesus Prayer each time he hit it. IIRC, the young boy kept smacking the tree with that stick for 2 or 3 days straight. Once the abbot saw that, he let the boy in to the monastery as a novice. And then promptly sent him to live in a hut up the mountain, tending sheep, fighting off wild animals, birthing lambs, sheering wool, etc.
In the modern world, it's extremely unlikely the abbot would give you his blessing if you just showed up out of the blue. They'd want to know who you are, have you visit a number of times, maybe stay for a couple of months or something like that to see how you do when the novelty wears off. But, basically, everything is 100 percent up to the abbot. He decides if you get to stay, when you become a novice, how long your novitiate will be (could be 6 months or 6 years...often it is around 1 to 3 years), what your job will be, how many prostrations you have to do in your cell, etc. Every monastery is a little different. Some are very different. So becoming a monk nowadays is often about finding the abbot and style of monasticism/community that fits you and to which you want to get "married," i.e. be in obedience to for the rest of your life.