Implementing StrStr()
Algorithm  


your implementation has some serious bugs and unexpected behaviour.  


If passing String.Empty as second parameter, it throws an IndexOutOfRangeException  
If passing the unique ending part of the first parameter appended by at least one character to the second parameter, it throws an IndexOutOfRangeException  

string test = "giladdarmonwhatareyoudoing";
int index = StrStr(test, "ng1");  

If you are passing null as either first or second parameter a NullReferenceException is thrown. Here it would be better to throw an ArgumentNullException by using a guard clause.  

if (test == null) { throw new ArgumentNullException("test"); }
if (strToFind == null) { throw new ArgumentNullException("strToFind"); }  


your implementation could be improved by  


checking if strToFind.Length &gt; test.Length  
checking if test.Length - i &gt; strToFind.Length  



Naming  

You shouldn't use hungarian notation. Consider to rename strToFind to searchForm or searchArgument.  

Refactoring  

After extracting the inner loop to a separate method, removing the now unneeded if (test[i] == strToFind[0]) and implementing the above we will get    

public int StrStr(string value, string searchArgument)
{
    if (value == null) { throw new ArgumentNullException("value"); }
    if (searchArgument == null) { throw new ArgumentNullException("searchArgument"); }
    if (searchArgument.Length == 0) { return 0; }

    int searchLength = searchArgument.Length;
    int length = value.Length;

    if (searchLength &gt; length) { return -1; }

    for (int i = 0; i &lt; length; i++)
    {
        if (length - i &lt; searchLength) { return -1; }

        if (IsMatchAtIndex(value, searchArgument, i)) { return i; }
    }
    return -1;
}

private bool IsMatchAtIndex(String value, String searchArgument, int startIndex)
{
    for (int j = 0; j &lt; searchArgument.Length; j++)
    {
        if (value[startIndex + j] != searchArgument[j])
        {
            return false;
        }
    }
    return true;
}  


I prefer the above, but you could also add  

if (length - i &lt; searchLength) { return -1; }  


inverted as condition to the for loop like  

for (int i = 0; i &lt; length &amp;&amp; length - i &gt;= searchLength; i++)
{
    if (IsMatchAtIndex(value, searchArgument, i)) { return i; }
}