Hi. I'm a daughter of Shinto priest... so I know little bit about Shinto Priest and Miko. My dad wanted to take over the Srine so he was expecting me to be a priest... (Thanks God, I got away!)
Since I was little, I had to be Miko to help my dad... Miko is like an assistant. Tradtionally, it is like Virgin something, right? But to me, it was like a waitress job... (not really working around food but)...
My dad's shrine is not so big so when there is a big festival, wedding or New Year's cerebration, we had to hire some girls for Miko position. We didn't really care who to employ as long as it they are female... Sometimes we hired boys but we don't call boys "Miko". To be Miko, you don't really need any qualification. On the ceremony day, Miko has to help Priest to so whatever the priest has to do... so you will learn by helping him so there won't be a training. (Not hard job... Like just use Sake when Priest ask you to, bring Tamagushi when priest ask you to... something like that) But if you are to work in a super big shrine, you might have to have some training to dance in front of Kami.
To be a priest... Easiest way (to your brain) is to be an apprentice under a priest for few years. You can be a priest after working under a priest everyday for 2 or 3 years (I think it was 2 years). But my dad went to Kokugakuin University to be a priest. I think there are only 1 or 2 University who have the course to be a Shinto Priest. Uni in Japan in usually 4 years and you really have to study history and tradition while you are at Uni (hard to your brain!!) Usually, it is hard to get into Uni but I could get in if I wanted because my dad could be my referee (Did I spell it right?)
By the way, what make you decide to be a priest??? So interesting... I was born in a Shinto priest's family and since I was born it was like... my life pass was already set and I hated it.
I think you have a chance to do Miko's job if you go and ask Shinto priest about a month before New Year's eve and say "Miko-san no Shigoto wo sasete kudasai" (Please let me work as Miko)or "Miko-san wo boshuu shiteimasuka?" (Are you looking for some people for Miko position?) But you need some Japanese speaking skills... because I have never seen a Shinto Priest who can speak English! Good luck!