How do I go about collecting past overtime wages from former employer whom told us 40+hours pays OT>>?

I just learned that in california overtime wages are supposed to be payed after 8hrs. I usually worked 10-12hrs per day 5-6 days a week. my dumb * belived the mngment when told that aftter 40hrs ot is paid. labor laws beg to differ , so how do i turn this info into $$$ ??? thank-u for ur help..