For each line in a file, read two integers and output the minimum
There are a few methods that will make the task easier for you:


File.ReadLines to read the file line-by-line (see also File.ReadAllLines)
Enumerable.Skip to skip the first line
Enumerable.Select to map the the lines to the minimum of the two numbers
string.Join for formatting the output


You might also consider taking the input and output file paths from the args parameter. Putting it all together (without error checking) could look like this:

public static void Main(string[] args)
{
    var minimums = File.ReadLines(args[0]).Skip(1).Select(GetMinimum);
    File.WriteAllText(args[1], string.Join(" ", minimums));
}

private static int GetMinimum(string line)
{
    var numbers = line.Split(' ').Select(int.Parse).ToArray();
    return Math.Min(numbers[0], numbers[1]);
}


I've been lazy here with error checking -- it would be a good exercise to add that to the code. It's also good practice to specify the file encoding when reading and writing files, which again I haven't shown here.