“初次登台”常见的英文表达是 “make one's debut” 。
例句：
She made her debut as a singer last night.（她昨晚作为歌手初次登台。）
The young actor will make his debut in the new play next month.（这位年轻演员将在下月的新剧中初次登台。）
此外，也可以用 “first appearance on stage” 来表达类似意思，不过 “make one's debut” 更为常用和地道。