Simply go to bangali classes . or u can learn at your own 
firts go with alphabeds then  learn some common bengali words and then try have a short conservation on the words u have learned with  any one who knows bengali..... watch different bagali movies or stories . Watch, listen, read and write in your chosen language.Don't be afraid to make mistakes while taking . u can also try Using Language Learning Software and Apps for learning  

~CHEERS~ hope it helped.....