There are a huge number of ways to do this: this tip shows a number of them for newline characters: Counting Lines in a String
] - but the principle is very much the same for whole words.
Probably the way you want to look is with a regex:
string text = File.ReadAllText(@"D:\Temp\MyText.txt").ToLower();
int hellos = Regex.Matches(text, @"\bhello\b").Count;
returns the number of times "hello" appears in the file as a complete word.