Which is the best tutorial available online to learn the recurrence relation concept and its solution in discrete mathematics, in a systematic way?
I liked this Discrete Mathematics: An Open Introduction, by Oscar Levin, for generating functions, so I'm guessing it will be good for recurrence relations.
Better yet, I very much liked generatingfunctionology by Herbert Wilf; it is the go-to text for what you are seeking. It addresses generating functions, and considerable help for understanding recurrence relations. (And it's free from the author, and downloadable as a pdf.!)
In addition, you'll find extensive information at Ronald L. Graham, Donald E. Knuth, and Oren Patashnik's Concrete Mathematics.
For beginner information, there are a number of sources and tutorials online. For example, see Brilliant.org, recurrence relations.
Another potential online source for introductory material is located here.