How to set errno value?

I have two calls to two different methods :

void func1() 
{
  // do something 
  if (fail) 
  {
    // then set errno to EEXIST

  }

}

And the second method :

void func2() 
{
  // do something 
  if (fail) 
  {
    // then set errno to ENOENT

  }

}

1. When I set the errno to some value , what does it do ? just error checking ?

2. How can I set errno in the above methods func1 and func2 to EEXIST and ENOENT

Thanks