Hey, UCLA undergrad here.
I'm posting off my phone so I'll have to be short but my advice is as follows...
The classes aren't weeder courses but they are definitely difficult, as any science class will be. What
@aaronrodgers said above is absolutely correct, don't overboard your classes for your first quarter. Maybe take Physics 6A, with LS2, and a psych class, or some other upper div class that you can take as an elective. There is a learning curve associated with the quarter system in the sense that it takes a little bit of time to understand how quickly things move. You also have to learn how to study - because if you're like most transfers I know, taking in material in a shorter amount of time can come as a bit of a shock at first.
Check out bruinwalk - see which professors are known to be better lecturers (more engaging) and take classes like that because the more immersed in the teaching you feel, the better you'll understand/grasp concepts.
Don't skip classes - this is the key to succeeding. The instant you fall behind, it starts killing you exponentially. You miss one day and then you lag on watching the podcasts and then it all gets harder from there.
As for your last question - there's no harm in teaching yourself some starter chem/physics - I don't know if it's absolutely necessary but it won't hurt you. The professors at UCLA really are great people and will help you if you look for the help. Go to office hours, be an active/engaged student, and make the most of your time at UCLA because really it was a FANTASTIC program and I wish I could repeat the entire process over again.
PM me if you have any questions - I'm happy to help where I can