Although this is not true for every single person, the most important agent of political socialization is the family.
As children grow up, they come to understand what political opinions their family holds. They come to associate this sort of political opinion with belonging to their family. Holding on to such opinions helps to make them feel that they are a part of their family.
In addition, the things they learn from their families help to shape their realities long before they start explicitly talking about politics outside the home. They come to understand their family’s attitude towards such things as taxes and government spending and crime. These attitudes become part of their basic values and influence their later political opinions.