The most straightforward way of attending post-secondary school without cost is to engage in a funding relationship with someone who will pay for your education for you.
In many ways, this is very straightforward: there are many governmental, private and npo / charitable organizations who contribute to student tuitions: the key is researching and finding one whose values and outcomes are in line with your own.
Education sponsors are organizations, programs or agents who will pay tuition obligations on student's behalf.
Canada
Rewards Foundation
Scholarships Canada
Scholarships.gc.ca
Forces.gc.ca
US
CollegeBoard
US military scholarships
AmeriCorps Education Award
Ed.gov
International
Hytes
Advance Africa
India Scholarships
2) Attend a free college
Some post secondary institutions are created on a model of no tuition cost to their students. Often these schools will have specific geographic catchments or entry requirements. This is a rare type of institution, typically characterized by one of:
Extremely high admission standards.
Specific education related agendas or highly stylized learning programs.
Patrons with interests in the college learning outcomes.
Current instances of this type of college:
Cooper Union
Berea College
Curtis Institute of Music
Polytechnic and Universities in Finland
Alice Lloyd College
Barclay College
College of the Ozarks
Deep springs college
Franklin W Olin College of Engineering
Saint Louis Christian College
Webb Institute
Bradenton
3) Seek training, not certification
Randy Pausch's Last Lecture (from CMU.edu as hosted on YouTube)
Many universities and college programs are available online, for free, to any students wishing to learn so long as that student is not seeking credit for the learnings.