first base vs first step

first base and first step both are nouns.

Word	Noun	Adjective	Verb	Adverb
first base	Yes	No	No	No
first step	Yes	No	No	No

As nouns, first step is a hypernym of first base; that is, first step is a word with a broader meaning than first base:

Other hypernyms of first base include initiative, opening, opening move.

first base (noun)	first step (noun)
the initial stage in accomplishing something	the first of a series of actions
the fielding position of the player on a baseball team who is stationed at first of the bases in the infield (counting counterclockwise from home plate)
the base that must be touched first by a base runner in baseball