This is an interesting post that appeared on Hacker News recently, in which a self-learner describes his path of study through what would amount to a very substantial undergraduate math major (probably bleeding over into some graduate study).
Mathematics for the adventurous self-learner
Equally interesting was the resulting discussion on HN, much of which was concerned with the question of what to study to prepare for this, namely learning math starting wite pre-algebra. The resources for this that received multiple recommendations were:
(1) Khan Academy
(2) The Demystified series of books by McGraw-Hill
(3) The freely available and peer reviewed books at OpenStax
(4) Your local community college.
My only experience with these is (4), which was my place of employment prior to becoming a stay-at-home dad. In my opinion, that resource does give a pretty good bang for the buck, but admittedly instructor quality can have a fairly high variance.