No single resource is good for all and serves all the purpouses. Here are links that are usefull when you are learning about DFT.
https://www.khanacademy.org/math/algebra-home
Khan academy is the resource when it comes to quickly and clearly introducing people to the topics of math.
Link above goes to algebra section of the page where you can find introductory material for all the topics
required to understand my DFT introduction.
dspguide.com
This webpage contains available for free (online or as chapters in PDF) version of a book
The Scientist and Engineer's Guide to Digital Signal Processing by Steven W. Smith.
This is the one that made me get what DFT is about. It contains all the theoretical introduction
to the topic of DSP along with its uses, but does not use a lot of math. Great resource for beginners.
https://ccrma.stanford.edu/~jos/mdft/mdft.html
Online version of a book Mathematics of the Discrete Fourier Transform by Julius O. Smith.
This is the resource if you are looking for math theory underlying the DFT. From explanation of complex
numbers, vector projections to DFT itself. It also includes proofs of used theorems.
Highly recommended after getting basics of DFT.