There is a pretty popular Yale course that you can take via Coursera for free (in the audit track): https://www.coursera.org/learn/negotiation. Its like a business school course so be prepared for lots of in depth case studies that you should really do to get something from the course. Good if you have at least some basic understanding of negotiations before you take the course.
Another course is this: https://www.coursera.org/learn/negotiation-fundamentals. It gives a step by step explanation and might be better suited if youre a complete beginner.