What political party should I join (and how did you choose yours)?

So, as you may know, I turned 18 in August, but I haven’t registered to vote yet. I have a California Voter Registration Form with me and I intend to fill it out soon. The parties available are: American Independent, Green, Peace and Freedom, Republican, Democratic, Libertarian, and Other.

How did you choose which party to join? I know it may be easy to choose between Republican and Democrat, but did anyone here join a third party? Why?

If you know me and my beliefs, you’ll know that I’m pretty liberal socially and environmentally. I’m less liberal economically. Political tests always label me as a left-leaning moderate or centrist. I’ll probably end up joining the Democratic Party, but I’m just curious for any last-minute info. Thanks. :)