when i become minster in my country

Depends on the country. In many countries (those following the Westminster model or the Nordic model), one gets to be prime-minister it the party he or she leads gets to win the elections (or, if the system uses proportional representation, and if no party has more than 50% of the seats in parliament, if you are the leader of the coalition that gets to form the government other countries (those following the semi-presidential model), the prime-minister is named by the president.

If you asked about being prime-minister as a career objective, getting into politics as soon as possible is a good idea and making yourself known is a must. You will need a very fast ascendancy in a party of your choice (note that it's a good idea to choose a large mainstream party with a lot of potential for the future). Furthermore, in many countries you are expected to be successful in a different domain than politics, such as business, but this is not obligatory.

The best way is to start by planning a career in politics. University subjects in the areas of Politics and Government are the best choice. Graduates of these courses can gain employment in a number of areas, such as Federal, State and Local government and councils, Electorate offices and Electoral commissions, Foreign Affairs, Public relations Consultants and many places within business and the corporate sector