If what you mean is "rigorously from scratch" then basic mathematics by serge lang is a great, but really challenging book which gives you an idea of what real pure math is, starting from the ground up.
You could even read an intro to set theory book if you're concerned with the very philosophical foundations of math, though I wouldn't recommend this to a presumed beginner and this is probably not what you're looking for.
If you just want to use math as a tool for engineering or physics or compsci or whatever then khan academy might be good but in reality you're probably a lot better in this regard than what you think you are. You're going to be inundated with things like factorisation and the quadratic formula; if you already have a grasp of algebra then you could start at calculus right away or whatever your major demands.
Either way, make sure you do eventually get started, make sure not to get caught in the trap of eternally looking for the perfect book/course, just get started, anything you choose is going to be good enough and the differences are minute.
