Nigeria has a document called the Constitution which sets out the rules on how Nigeria should be run. You could say the constitution is the country's rule-book. Lots of countries have a constitution document that does the same job. Nigeria has had four constitutions since independence - the current constitution was made in 1999, the previous constitutions were made in 1979, 1963 and 1960. You can find a copy of Nigeria's current constitution here:
Nigeria's constitution splits the running of the country up into three arms. These arms are: the "Executive', the 'Legislative' and the 'Judiciary'. Each of these 'arms' or groups of leaders, has a lot of power to influence the way the country is run, but they each also serve to monitor the other arm to make sure that they do not start to misuse the power they have been given.