What do you need to do to become a republican or democrat?

There are different levels of involvement with the Democratic and Republican Parties, depending on how much time you wish to devote, and how passionate you are:

Most states allow voters to register as affiliated with a political party. Voters can either declare their party affiliation when they first register, or obtain a party registration form from the local (usually county) election board. Registering with a party allows a voter to take place in nominating primaries and caucuses, where the candidates for general elections are chosen. Most state parties have closed primary systems, where only members of the party can vote to choose a nominee. Some states have open primaries, where voters can vote in either primary. Some states allow un-affiliated voters to declare a party at the polling place. The political parties set the rules for primary elections and caucuses, so check with your local party.

If a voter is looking to become an active member of a political party, both the Republicans and Democrats offer membership on exchange for dues. The parties use the dues for campaign funds and administrative costs. As a member, you have opportunities to respond to polls which let party leadership know what is important to the rank-and-file, run for party leadership, get advance notice of campaign events, and possibly meet political figures. Members often receive special offers on campaign merchandise, such as buttons, bumper stickers, and t-shirts.

For more information, contact your local (county or municipal) party club, your state party, or the national party committees (Democratic National Committee/DNC and the Republican National Committee/RNC)

Note that, in addition to the Democratic and Republican parties, there are a number of other parties. These are typically called "third parties", but since there's many more than three total parties, a more accurate term would be "minor party". Most minor parties are ideologically further right or left than the two major parties, as the major parties tend to overwhelmingly win moderates. Most of these parties have limited or local followings; perhaps with the exception of the Democratic Farm-Labor Party of Minnesota, which takes the place of the Democratic party in the state (DFL elected officials caucus with Democrats on the national level).