Strong desire to serve/ordain

Householder, interested,

- Namo tassa bhagavato arahato sammā-sambuddhassa -

One who starts to serve worthy things, outwardly and inwardly, starting with outwardly, is sure to increase Upanissaya, sure to arrive there of what he/she had served. Having, doing not serving of what is worthy to, how could one ever arrive, ever ordain/attain? It would simply die out for one, inwardly and outwardly soon.

As soon as such desire, desire for going forth under the tripple Gems for liberation from suffering, wholesome mind, kusala mind arises, go after it, don't wait a moment, no need to worry much on failure or to be perfect before going to train at the highest school.

What ever age, what ever condition, seek for it, as soon as it arises, and for all who desire are ways to arrive there, at the holy live and eventually liberation from suffering, desire leaving home, leaving stands.

Three things are praised by the wise: giving and help for parents (serving) and renouncing, going forth.

Be quick! (the story of Veen. Maha Kasspa and his Ven. former wife in previous life) Seek and take the possibility if such desire arises and avoid what ever hindrance and people telling otherwise.

Be quick in doing what's admirable. Restrain your mind from what's evil. When you're slow in making merit, evil delights the mind. dhp

All you could lose is to return where so long, long returned, again and again. So this path has even a "payback-quarantine" if it does not work for you yet, missing Upanissaya, something that needs to be nourished and gone for, again and again, till met.

Others then many advises (sadly even monks and those who should be informed, appearing as teacher), the Buddha did not allow his monks to teach much in regard of Vinaya (rules for monks) for one who desires for going forth. So such is only good if wishing to prepare yourself meanwhile, till finding the possibility.

Look also at: How to ordain as monk?

The inspiring story of Yassa and many others after him here: Mv I 07: Pabbajjākathā — The Discussion of Going-forth

And don't fear possible disrobing, as you could try anew with more inclination for seven times.

Mudita

(Note that this gift of Dhamma is not dedicated for trade, exchange, stacks or entertainment but as a means to make merits toward release from this wheel)

Some times monastic life may not be calm as you think. Also austerity of vinaya will make it harder than lay life incase your have significant attachments.

So best is:

1. lean the Vinaya so you do not walk in blindly
2. tame your senses in lay life so you can make your ordained life more productive and live according to the Vinaya

Also many monks do disrobe. I don't think this should be taken as failure. You are just re prioritising certain things in life as you were perhaps not ready to have been a monk.

If you do enter the order do it in cordial manner so that you do not burn bridges and create any disharmony so you can continue life if your in case your monastic life does not work out.

Also one requirement is that your parents should approve it.

I would say to give it a test run. Start keeping the 8 precepts:

1. Panatipata veramani sikkhapadam samadiyami I undertake the precept to refrain from destroying living creatures.
2. Adinnadana veramani sikkhapadam samadiyami I undertake the precept to refrain from taking that which is not given.
3. Abrahmacariya veramani sikkhapadam samadiyami I undertake the precept to refrain from sexual activity.
4. Musavada veramani sikkhapadam samadiyami I undertake the precept to refrain from incorrect speech.
5. Suramerayamajja pamadatthana veramani sikkhapadam samadiyami I undertake the precept to refrain from intoxicating drinks and drugs which lead to carelessness.
6. Vikalabhojana veramani sikkhapadam samadiyami I undertake the precept to refrain from eating at the forbidden time (i.e., after noon).
7. Nacca-gita-vadita-visukkadassana mala-gandha-vilepana-dharana-mandana-vibhusanathana veramani sikkhapadam samadiyami I undertake the precept to refrain from dancing, singing, music, going to see entertainments, wearing garlands, using perfumes, and beautifying the body with cosmetics.
8. Uccasayana-mahasayana veramani sikkhapadam samadiyami I undertake the precept to refrain from lying on a high or luxurious sleeping place.

If you are able to keep them for a while without too much difficulty and you feel you can live a life of renunciation for a long time with contentment, then you will know you will be able to ordain.

In the meantime you should try to pick out the monastery you would like to consider ordaining at. Ideally you want to establish a relationship with the monks there first. If you want to be with monks who practice a particular meditation method then find such a group, and if you want to be strict in upholding the vinaya then make sure to find strict monks.

This is a great question and the answers here (from Bakmoon and Suminda) are wonderful. I'll complement with something from my personal experience.

"how can one determine ones decision to ordain, whether it's a strong volition or not"

It may be that going forth ultimately is the single thing one can do to know the true nature of his disposition. But since disrobing may come with a lot of practical difficulties, taking precautions is very valuable: become used to a life without luxuries, familiarize with the monastery, monks and retreats, etc.

But giving the ear to advices may be troublesome, depending on who one seeks out to get advices. There is a common discourse that people looking to become a monk often hear in the west: "you are running away from [blank]".

It may be more articulate than that, when it focuses on debunking the romance of becoming a monk: the idea that things will be so beautiful, you will have 100% of the time to meditate (or do prostrations, or read suttas, or perform all rituals you fantasize about, and wisdom will come quickly, and your teacher will be so and so, etc).

Or it may focus on the idea that a monk's life is easy and therefore, not very fruitful for evolving one's practice: "the lotus grows in the mud, so you should do your practice in the real life, not in the top of a mountain", they say.

On one hand, these are very unhelpful advices, and very disconnected from what monastic life is, and from what buddhism is -- at the minimum, from this rhetoric, no one is suited for monastic life, or should become a monk ever (it is also a little disrespectful, drawing an image of monks as cowards).

On the other, there is a risk of fantasizing with a romantic idea of being a monk and there is a risk of using the going forth as an excuse. I think the best strategy for the first is what others have said here, try to become familiar with monastic life, in order to dispel any fantasies you may have. The antidote for the second is to try to identify if the intention of going forth is the going forth itself, and not just the relief of not having to deal with "[blank]" anymore (its ok to feel good about not having to deal with "[blank]" anymore, but I think this should not be the reason for going forth).