There are three basic methods by which an individual may become a candidate for president of the United States.
Presidential nominees are selected by delegates at national nominating conventions. Individual states conduct caucuses or primary elections to determine which delegates will be sent to the national convention.
Independent presidential candidates typically must petition each state to have their names printed on the general election ballot.
https://ballotpedia.org/Ballot_access_for_presidential_candidates#Party_nomination_processes