Help me with triangle.

I have tried fixing up your code:

using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            int a, b, c;
            Console.WriteLine(" a: ");
            a = int.Parse(Console.ReadLine());
            Console.WriteLine(" b: ");
            b = int.Parse(Console.ReadLine());
            Console.WriteLine(" c: ");
            c = int.Parse(Console.ReadLine());
            if (a <= 0 || b <= 0 || c <= 0)
            {
                Console.WriteLine(" All values must be great from 0!\n");
                Console.Read();
            }
            if (!(c < a + b) || !(a < b + c) || !(b < a + c))
            {
                Console.WriteLine("Triangle no exist!\n");
                Console.Read();
            }
            else
            {
                Console.WriteLine("area triangle = {0}", Heron(a, b, c));
                Console.Read();
            }
        }

        static int Heron(int a, int b, int c)
        {
            int p = (a + b + c) / 2;
            return (int)Math.Sqrt(p * (p - a) * (p - b) * (p - c));
        }
    }
}

The things that were 'wrong':
Console.WriteLine << " a: ";
Seems like a mix between c# and c++ syntaxis, it should be something like this:
Console.WriteLine(" a: ");

Console.ReadLine >> b;
Should be something like:
b = int.Parse(Console.ReadLine());

static void Main(string[] args);
//...
int main()
{
    //...
}
The entry point (main function) should be defined as:
static void Main(string[] args)
{
    //...
}

system("pause");
To pause the execution and wait for the user to press a key, try:
Console.Read()

return 1;
Since the main function returns void, you don't need to return 1 or 0.

return sqrt(p*(p-a)*(p-b)*(p-c));
sqrt is located in the Math class so it should be:
return Math.Sqrt(p*(p-a)*(p-b)*(p-c));

Also it should be cast to an int:
return (int)Math.Sqrt(p*(p-a)*(p-b)*(p-c));

int Heron(int a, int b, int c)
And lastly all function used in a console application should be marked as static
int static Heron(int a, int b, int c)