WordNet sense 1 (the time at which something is supposed to begin):

beginning, commencement, kickoff, offset, outset, start, get-go, showtime, starting time

WordNet sense 2 (the first or highest in an ordering or series):

number one

WordNet sense 3 (the first element in a countable series):

number 1

WordNet sense 4 (an honours degree of the highest class):

first-class honours degree

WordNet sense 5 (the lowest forward gear ratio in the gear box of a motor vehicle; used to start a car moving):

low, first gear, low gear

WordNet sense 6 (being or denoting a numerical order in a series):

ordinal

WordNet sense 7 (used of the first or originating agent):

prime

WordNet sense 8 (first in rank or degree):

premier

WordNet sense 9 (preceding all others in time or being as first made or performed):

original

WordNet sense 10 (first in order of birth):

eldest

WordNet sense 11 (occurring at the beginning):

initial

WordNet sense 12 (serving as a base or starting point):

basic

WordNet sense 13 (serving to set in motion):

inaugural, initiative, initiatory, maiden

WordNet sense 14 (first or beginning):

opening

WordNet sense 15 (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)):

first base

WordNet sense 16 (prominently forward):

foremost

WordNet sense 17 (ranking above all others):

world-class

WordNet sense 18 (of the transmission gear causing forward movement in a motor vehicle):

forward

WordNet sense 19 (the initial time):

for the first time

WordNet sense 20 (before anything else):

firstly, first of all, first off