The selection process for US Presidential candidates is complicated but we are here to help!

For Presidential elections, which happen every four years, US candidates must first put themselves forward for primary votes for their party in each state. Their admission into the election in each state depends on the candidacy rules of each state. The candidate who wins in the primaries for their party is the one who receives the most votes from delegates. When a candidate wins a state for either of the main parties, they receive votes from the pledged delegates of those states. Superdelegates on the other hand can vote for whomever they please, although it is uncommon that they do not vote for the candidate from their party with the most primary votes. 

The two winning candidates from either party are then announced at their respective conventions. In some elections, a third-party candidate is allowed to run in the elections.

Congressional elections happen every two years. Candidates for each constituency can nominate themselves or are nominated by their party. There are only party primaries if there are multiple candidates for a constituency. The rules for primaries vary depending on the state.