be/get in on the ground floor idiom
从一开始就参与（某事）
to be or become involved in something from the beginning
He was sure that he was getting in on the ground floor with the next big thing. 他当时就认为自己开始参与的是新一轮的热点事件。
Can someone tell me how you get in on the ground floor of a deal like that? 有谁能告诉我你怎么才能一开就参与那样的交易？