How to find the time complexity of an algorithm?
You can check out the easy ways here
easy ways for complexity calculations.
Start with tutorial point good for beginning.
Tutorial point Data Structures and Algorithms
Learn all notations for Complexity from geeksforgeeks or some website.
Then you have to learn in dept to understand complexity calculation of complicated algorithms.
Please read some standard books according to your curriculum to avoid getting confused with online material ( notations may vary from source to source etc.).
Try these Courses from Coursera after reading at least one book on algorithms:
Stanford University: Algorithms: Design and Analysis
Check this article out