What are good resources to learn the basics of graph theory
I can think of two very good resources that may help you to acquire the foundational knowledge on graph theory and complex networks:

Network Science by Barabasi is you prefer a web tutorial: http://networksciencebook.com/
The Structure of Complex Networks by Estrada if you prefer a book: https://oxford.universitypressscholarship.com/view/10.1093/acprof:oso/9780199591756.001.0001/acprof-9780199591756