Help in learning trees and graphs

Hii… I began learning about binary tree few days ago, I understood some of the algorithms used. But I am unable to understand how we will apply those in contest.
Please can anyone tell how we can apply this in contest and using java as a language as I am unable to understand how I am going to implement this in the contest using java classes and collections.
Also, can anyone help me to guide how to approach towards learning trees and graphs.
Thank you.