There are many courses available for free on Coursera and Edx to learn algorithms. Also there are a couple of introduction to algorithms from Thomas Cormen (one of the authors of the wellknown CLRS book):

https://ocw.mit.edu/courses/electrical-engineering-and-compu...

https://ocw.mit.edu/courses/electrical-engineering-and-compu...

Also, take a look here:

https://teachyourselfcs.com/#algorithms

Have fun!