Answer:
The media
News outlets will often have a list of who is running for office and their platforms.
Voter's pamphlet
When registering, voters are often given a pamphlet that has a list of the candidates running in the upcoming election.
Online voter registration
Many states have an online voter registration system where voters can view a list of candidates.
Sample Ballot
Some states have a voting website where voters can view a sample ballot of the upcoming election. This website will often list the candidates running for office.