Learning how to hack a website

I wish to learn how to hack websites (finding Admin username/password) by working through a specific example of a website I have.

I could contact a "hacker for hire" on the Net (there is no lack of them…) and ask him/her to do the job, but I am not interested in doing this : I want to get the know how and learn how to do it by myself.

I have already gone through several tutorials (including some on YouTube). Some are more interesting than others. I have also tried the SQL injection method, using the Havij and SQLMap tools, but, apparently, my website is not "injectable".

Could someone suggest any step-by-step tutorial or online course I could follow in order to learn efficiently how to hack my website ?