Bill Shillito's lecture series for Project Polymath is by a mile, the best introduction to higher level mathematics. It requires absolutely no prerequisite knowledge.

Introduction to Higher Mathematics: https://www.youtube.com/playlist?list=PLZzHxk_TPOStgPtqRZ6Kz...

If you find he talks a bit slow, change your playback speed to 1.5x. Enjoy! :-)