I'm underage, I want to get castrated and become gender neutral. Where I should go and what do I need to do before the procedure?

Since we don't know what country you are in it is hard to say. In the US being underage means it is not going to happen. Also you would need to talk to a psychologist before having such a surgery. Because it is permanent and a life altering surgery most if not all surgeons will not do such a surgery unless you have been confirmed to fully understand ALL the repercussions.

My best advice is for now find a psychologist to work with now and start saving your money so when you do become of age you are that far along.

The key word in your question is underage. You can't.

Join a monastery instead. Look  on the internet... There's lots of monasteries ready to take  young men who fit the way you feel, and you can contribute to mankind by doing good deeds at the same time.

Being "gender neutral" is the least of the effects your orchiectomy will have on you.

You will also be asexual whether you wanted to be or not.

And you will also be without the hormone testosterone---which will result in all sorts of undersirable effects in your body, now and in the future.

Don't do it, the lack of testosterone will cause you problems you probably did not think about - being low can cause depression, muscle aches, joint aches, insomnia - look up  "low T" symptoms, and you could get all of them.