What is a word for someone who is totally new in something or participates in something for the first time?
The obvious one is novice: "A person new to and inexperienced in a job or situation" (Oxford)
Neophyte is an alternative: "A person who is new to a subject or activity" (Oxford). To me that emphasises newness over inexperience, so it's a bit closer to what you're looking for - though it's quite an obscure word so I'd be cautious about using it.
But neither quite implies "for the first time". If it's important to stress that it literally is the first time, I'd just be explicit and say first-timer (or first-time participant, etc.)
If you want to highlight the 'first-timer' part...
- A person who is naive, innocent, or inexperienced in a particular context:
he’s a political virgin
Abecedarian, as a noun, means means either a novice — a person just learning the a-b-c's of something — or it can describe something arranged alphabetically.
- Say abecedarian and you will notice something peculiar — it sounds like a-b-c-d, and that’s not a coincidence.
- a child learning how to spell.
- a person who is starting to do something or learn something for the first time:
- This judo class is for beginners only.