it's my first timeUsed after a noun or verb phrase to say that something is happening for the first time. The topic particle 은 changes to 는 after a vowel. In 처음부터, 처음부터 is a separate expression meaning “from the beginning.”
KO: 한국 음식은 처음이라 조금 매울까 봐 걱정돼요.
EN: It’s my first time having Korean food, so I’m worried it might be a little spicy.