OK, it sounds like OP wants to know the intricacies of how presidents are chosen in the U.S. So the first thing is that the legal requirements are that a person must be a natural-born citizen (the meaning of "natural-born" has never been fully elucidated by the courts) be at least 35 years old and have lived in the U.S. for at least 14 years. That's the technical qualification for someone to be president. The political process and practical aspects are a little more complicated. The U.S. has a two-party political system. we have 3rd parties, but 3rd parties candidates are not elected for office very often and tend to be more ideological than either of the two parties. 3rd parties are a way that people sometimes express grievances with the political system when they aren't doing well, act as a "spoiler" in elections, or act as an ideological influence on the two parties. Additionally, there are caucuses which can act as a party within a party as a way to support a specific interest or version of party ideology.
One thing you have to keep in mind though is that a legal, highly formal system of bribery permeates the entire political process. This can make or break a candidate and determines who's interest they act in when there is a conflict of interest between the political donor class and the "average citizen." Even in the rare instances when the donor class loses a political battle, the loss will usually be mitigated in some way.
Back to the selection process. People who want to be president must get the support of party leadership, already have some money, get the approval of donors (which means getting donations), and get endorsements from various ideologues and king-makers, and various media outlets. This enables the candidates to run campaigns with expensive advertising and non-stop visits to different areas around the nation, particularly states that vote early in the primaries. The campaigns try to get voters registered in the candidates respective parties to vote for them in the primary elections. The primaries are intra-party elections where voters who are party members (one becomes a party member simply by checking a box on the voter registration from) select the candidate that the entire party will stand behind and get party resources for the general election. However, the U.S. has low voter turnout in general elections and even lower turnout for primaries, meaning that people who vote in primaries are often strongly ideologically committed to a certain political outlook, sometimes to the point of extremism. The candidates must appeal to these voters by taking strongly ideological positions (reality, practicality, and legality be damned), stay in the good graces of party leaders, and receive continued support from the donor class (which has different priorities than the voters) to win the primary and receive continued support after the primary.
The timeline to U.S. elections goes something like this. Shortly after a presidential election, the losing party will begin quiet discussions about the next election and people who want to run will begin testing the waters to see if they have a chance, this intensifies after the midterm elections and people who want to run will indicate that they are considering a presidential run. From that point to a few months before primary season, lack of financial support, lack of voter interest, and scandal will narrow the field down to a few candidates. Right now we are at primary season, which starts this Tuesday and runs until June (although a candidate emerges much sooner than that). After primary season, the delegates selected for the candidates in the primary elections will vote at the party's political convention shortly after primary season ends.
The winning candidates are now in the general election season. To win the general election, the candidates must appeal to a wider segment of the voting population, receive continued financial support, and still appeal to the party. More specifically, the candidates must persuade independents, low turn-out voters, and people not yet registered to vote for them in the general election. This is most important in swing states, these are states with a similar number of Democrat and Republican voters, and have a history of voting for both Republican and Democratic candidate over the last couple decades or where a candidate doesn't receive an overwhelming percentage of the vote. These are Florida, North Carolina, Virginia, New Hampshire, Pennsylvania, Ohio, Iowa, Wisconsin, Minnesota, Colorado, and Nevada. Because most states are safe for a certain party, and only a handful of states are not safe for either party, a relatively small percentage of the population effectively decides the outcome of an election. In the election, people really vote for electors, who make up the electoral college, which actually votes for the president. Except for Maine and Nebraska, it is a winner-take-all system in each state.
Sometimes candidates can bypass the donor class and become serious contenders if they are self-funding or raise many small donations. These candidates run on a platform of economic, political, or cultural reform and come in both reactionary and progressive flavors and may or may not be members of either party and advocate for positions that are in conflict with the wishes of the donor class. This is basically the political phenomena of Trump and Sanders. These voters tend to believe the entire political system is corrupt and they have gotten a raw deal as a result of unholy alliances between far-away business, government, and sometimes cultural institutions. However progressives and reactionaries are different on what they see as the exact nature of the problem and what the solution to those problems are.
