How according to "If" can one achieve everything on earth and become a man?
Your question refers to the last two lines of this famous and brilliant poem, which act as the culminating idea of the entire poem and all the things that are suggested as being the qualities and hallmarks of being a "man." If, the poet says, you can do all of the things that I have listed, then:
Yours is the Earth and everything that's in it,
And - which is more - you'll be a Man, my son!
After all of the separate scenarios and qualities that are highlighted as showing you are a man, this represents the poem's concluding thought. There is of course a sense of hyperbole in this ending, as it equates being a "Man" with possessing the earth. What I think Kipling is trying to get at is that the kind of qualities that his various images of what being a man is all about present us with are the kind of qualities that we need if we are to mature and live a successful life that is not characterised by moral failings. There is something in Kipling's poem that captures the essence of what it is to be a successful man who is so confident in his own abilities that he does not allow himself to be swayed by others unduly. This is the route to becoming an actor on earth rather than being a passive, helpless individual, and therefore metaphorically owning the earth.