How to study effectively?

Have a look at the Pomodoro technique.

I think I can help out.

Don't set yourself time-related goals. Don't say 'I want to revise for X hours'. Set yourself content-related goals. For example, I have a standing goal that I will progress through 15 pages of my History textbook a day. In setting yourself content-related goals, you work more efficiently because you won't be thinking 'Just a little bit longer till I can stop', and you'll actually be able to measure what you've accomplished.

Thank you both. I followed both what you said and I have already gone through 4 chapters of my book yesterday and today.