The people in a particular land mass may want to declare themselves as a country if they so desire. But this is not an easy feat. First, they would need to declare independence from whichever country they are a part of now. This is part of the international laws set up by the Montevideo Convention on the Rights and Duties of States, adopted in 1933.
The second step is to be recognized by the countries that currently exist in the world. There are some countries that may not accept the independent stand of several nations if they wish. The third and final step would be to join the United Nations as a member country. These are important requirements for a place to be called a country.