I get this question many times,so thought of creating a post about how i learned ..
I found this book T-SQL Fundamentals to be very helpfull. Itzik Ben-Gan is worlds renowned expert on TSQL and he also wrote a number of books. I strongly recommend exploring other books by him
After completing the book ,i started reading Training Kit (Exam 70-461) by same author.Finally i attempted exam and passed.
In this learning process StackOverflow helped me a lot.I strongly recommend creating an account and start asking questions.I also recommend checking this link prior to asking questions,so that they wont be downvoted How to post a T-SQL question on a public forum
Also you can download SQLSERVER community edition for free here and start practicing
That was pretty much ,how i started learning TSQL.Sharing this in case,it helps some one