"So as a newcomer in LA as of this upcoming Monday, where would one find out about worthwhile open call auditions?" - @ActorAdamRay
"Worthwhile open call" can be a bit of an oxymoron in Hollywood, so I'll just focus on finding worthwhile auditions in general.
For on-camera auditions - indie film, student film, commercials, industrials, etc. - create online resume profiles on Actors Access and LA Casting and start submitting yourself online. NOW Casting and CAZT are up and coming online submission services that are worth checking out too.
And after you sign up, make sure people can find your online resume link!
If you're more interested in stage work, definitely sign up at Backstage.com. You can subscribe to their print edition or save a tree and get an online account to view a wealth of casting notices. They've just redesigned their site and it looks amazing!
Some theater open calls are definitely worthwhile, though know that dues paying membership theater is a thing in LA, meaning sometimes you have to pay the company to be in their shows.
And if you're interested in background work, sign up at Central Casting. Extra work can be a fun way to meet actors, learn about set etiquette, and get paid to stand around. There are call-in services you can use to get even more extra work too - Joey's List is one that a friend uses.
Finally, I always suggest actors create an action plan for their career to hone their focus and keep from feeling overwhelmed. Remember, it's a marathon, not a sprint. There's a ton of opportunity in Hollywood, so go for it! Break a leg!