What is the most efficient way to fix the time complexity of an algorithm?

I have looked through Google and resource for a clear and straightforward explanation of how to calculate time complexity, but I have yet to find one.

What information do I already have?

The following code is:

Code:

char h = 'y'; // This will be executed 1 time
int abc = 0; // This will be executed 1 time

The following loop is like the one below:

Code:

for (int i = 0; i < N; i++) {
    Console.Write('Hello, World!!');
}

This will be executed only once.

• int i=0; This will be executed only once.
  

The time is actually calculated to i=0 and not the declaration.

• i < N; This will be executed N+1 times
  
• i++ This will be executed N times
  

The number of operations needed to complete this loop is {1+(N+1)+N}= 2N+2.

I am familiar with the basics of time complexity calculations but have seen complexity measured in terms of O(N), O(n^2), O(log n), O(n!), and other forms.