I recommend watching the following videos for learning about graphs. These are really well made videos and he covers a lot in these short videos, so if you feel overwhelmed, use the rewind and pause buttons.

Introduction to Graphs

What are graphs? What are different types of graphs (directed and undirected, trees and DAGs, etc)? And examples of using them to model web links, flights between cities, and other things.

How to represent graphs?

How to store (or represent) a graph in code? i.e. adjacency list and adjacency matrix. When do you use one vs the other?

Further reading

The following text tutorials on csacademy are quite simple and high quality. Here are the links: Introduction to graphs and Graph Representation.