Primary things are the first things one should focus on or do because everything after that is based on what happened in that beginning stage.
Elections are based on a similar process. People choose to become candidates for an office and begin a campaign, sometimes with party support and sometimes without party support.
The next stage is the party conventions in which the party often selects the candidate they will endorse. The other contenders might honor that process and drop out, or they may have reasons they consider to be important enough to continue on in the race.
The primary election is the next stage. In Minnesota each party runs a set of candidates who identify with that party and from which voters will decide who will be the candidate of that party in November. Of course, a candidate can continue to run as an independent or with other affiliation, but that is not as likely.
In the November election the basic contest is usually between two candidates, one from each major party. If you are going to vote, those two are the only options from the two major parties.
The reality is, like most things, if you want to have much influence in the process, you have to show up early. The earlier you show up and participate the more influence you will have. If you are a party insider you will have early influence. If you are, like most of us, just ordinary voters you will have the most influence by voting in the primary election since that is when the party candidate is selected.
If you wait until the general election you will only have a choice between the candidates selected by others within the two major parties.
Get involved in the primaries.