Just read a book like "University Physics" by Young/Freedman or "Fundamentals of Physics"
by Halliday/Resnick (not the 9th edition!). At the end of this book there is a section on
modern physics introducing relativity, quantum theory & cosmology at just the right level
for someone who has studied all of the previous classical mechanics & Electromagnetism in
the book. Once you have finished either of these massive books you'll be in a position to
have a look at some proper quantum theory without that horrendous feeling you get from
skipping details in a math book (which gets increasingly worse as you read on!).
Since both of those books require calculus what you could do would be to read a different
book called
https://www.amazon.com/dp/0201473968/?tag=pfamazon01-20, I've never read it but it looks perfect if you
mix it with a calculus book like Stewart's calculus (or cheaper & way better -
https://www.amazon.com/dp/0871503417/?tag=pfamazon01-20 ($0.03!))
If you do not know what dx/dt means inside out stay away from Lagrangians!

You'll learn what dx/dt means properly in the Swokowski & Integrated Physics/Calculus
books
I'll also recommend
www.khanacademy.org if you need help learning
your calculus, you could even watch the calculus videos now & find
out what dx/dt means
