How to start learning formal language theory
Theory of computation by M. Sipser is interesting per se. For introduction you have to practice and go in following sequence. 


Regular language and automata 
Context free grammar and expressions 
Pushdown automata
Non context free grammar 
Lr(k) grammar 
Turing Machines 


If interest level drops keep reading a fun book 
https://en.wikipedia.org/wiki/A_New_Kind_of_Science