You could use an app called Forest. But I think the most important thing is your motivation.

Get a picture of an anime character you look up to or draw it yourself and place it in a photo frame. Then place it on your desk so that it is looking at you. Imagine that if you lose focus you will never get to their level.

EDIT: You can also look at it this way. Take someone who is better than you. When you are studying and use your phone for 15 minutes just remember that person is now 15 minutes ahead of you.