MI is not a fun place to get stranded in, unless you're in A2. Trust me, born and raised, and stuck, and re-stuck, and re-stuck here, time and again.

And there's a whooolllee big country between MI and Cali, so yes, you need to figure out how to do this, before you get to the land of honey and nutz.

Where in MI are you? That would be more helpful, to figure out where you are first...
Like Cristian said, hopping trains isn't always that easy (and is damn dangerous and illegal, if you don't know what you're doing - I've never done it myself, basically cuz I haven't found anyone to hop with yet). However, I know that there are shit-tons of trains in Chicago; it's a major hub to all points west, and I'm sure you could find more crusty/hoppin' kids down there that could point you in the right direction (as far as I know, there ARE a few trains that go directly to Cali from Chicago?) If you could Greydog it to Chi-town, you'd probably have better luck, hopping or hitchin...
Hitchhiking - now, that's a whole 'nuther story. Do you have a partner/person to go with you? I've hitched short distances by myself between (a couple states), but you never know what kind of whackos you'll meet on the road, girl... No matter how strong and determined you might be, it's ALWAYS good to have someone to go with, ESPECIALLY if it's your first time. I generally hitch the easy/'legal' way, which involves a lot of sitting at truck stops with a sign while practicing juggling, reading, writing, or talkin' story. You CAN actually go out and sit on a ramp, I suppose, but MI cops are generally DICKS, and would probably get dammmnn excited about seeing hitchhikers, since they're few and far between in these parts. It depends on where you are, to determine what route you're going to take... Hopefully, you're capable of looking at maps.
Anyways. I'm in Muskegon, shitty-ass little town on the west sydde. I highly doubt you're here. haha. But I have friends around the state, that might be able to at least give ya a floor to crash on, perhaps. Living outside = tarps are your best friend. Tarps preserve body heat just as (if not better) than sleeping bags. If you have both, get in sleeping bag, then roll up in tarp. Garbage bags work almost as well. It's getting warmer out, though, so you're in luck...
Anyways. Best of luck!! If you have any questions, or at located anywhere remotely near me, I'd be happy to hang out, maybe even catch out with you... I'm looking to get to Cali myself, but am in a strange, cluster-fucked famdamily situation a.t.m....