Wikipedia is a fantastic resource, but it is first and foremost an Encyclopedia, a reference. It has many times helped me refresh my memory or helped explain something I read first in another context. But it is not a good place to learn a brand new subject.

There are many good resources to learn math, most in the form of books (some of which have digital versions). Personally, I find it best to find a subtopic that interests me and dive in. Though I will say that I would recommend "Chapter Zero" to most people wanting to get back into math after a long break.