There's no way to find out specifically how you ended up on spam lists. The two most common are: 1) Your email address was sold by a company you signed up with. 2) Forwarded emails.
Number one is easy to understand. Number two makes sense once you think about it.
What happens is, someone sends a joke email to you and twenty of your friends. Now, your email address is on twenty computers. Four or five people think it's funny, and they forward it to twenty of their friends. Now, your email address is on around one hundred computers. Notice that you have not done anything. Everything is done by other people. More people forward the email, and by default, most email programs include the entire distribution list each time it's forwarded. Pretty soon, without you doing anything, your email address is on thousands of computers of people you have never heard of.
Eventually, it gets forwarded to a computer that is infected with a virus that sends spam. They harvest your email address, pass it around to other machines on that botnet.
Once the spammers have your email address, there's no way to remove it. They're jerks that way.