Answer:
First, you make a personal decision about which one you support (people have all sorts of reasons for making this choice). You don’t have to tell anyone what your party affiliation is if you don’t want to.
When you register, you can declare your political party if you want to. One advantage to registering as a democrat or a republican is that you can participate in the primary or caucus process to help select the presidential candidate that will represent that specific party (This process varies by state and a few states have open primaries where undeclared members of the public can vote, too.).
At any time you can change your mind. If you said “I’m a republican” last year, you don’t have to stay a republican forever. At the next election you can switch and say, “I’m a democrat”. Also, there are additional parties besides republican and democrat (green, libertarian, etc.).
Some people are independents. They don’t associate themselves with any specific political party and for each election they listen to the candidates’ stands on issues in order to make a voting decision. So, you don’t need to join a political party in order to vote.
Step-by-step explanation: