(PW) start from the beginning so you’ll have full advantage of any favorable outcome.

He is a very wealthy man.  He was one Microsoft’s first employees and got in on the ground floor.

(UsingEnglish)

If you get in on the ground floor, you enter a project or venture at the start before people know how successful it might be.

(free)

get in on (or start from) the ground floor Informal

a.  to enter a business, organization, etc., at the lowest level

b.  to be in a project, undertaking, etc., from its inception

(your)Informal

The beginning of a venture, especially regarded as a position of advantage: investors vying to get in on the ground floor of a development project.