上个月我成为了足球队的一员用英语怎么说

Last month, I became a member of the football team.
This phrase comes from the verb "become" which means "to start to be" or "to change into" a certain state or condition. The prepositional phrase "of the football team" is used to indicate that the person is now a part of that particular group.
Examples:
She became a member of the book club last week.
He became a member of the band after the audition.
Translations:
她上周成为了读书俱乐部的一员。
他在试镜后成为了乐队的一员。