Well, you post is really too SHORT! To give any useful answers: when? how long in each place? budget? do you plan to share a room? how are you getting there?

Off the top of my head, LA without a car will be difficult-ish. There are lots of great areas you could stay in without a car, but getting from one of them to the other is going to be hard.

Santa Monica (great place to be) to Pasadena is a little over half an hour in a car, but by public transportation it is between an hour and a half and two! Hollywood is about the same amount of time, Beverly Hills about an hour. On the other hand, you can bike from the Pier in SM to Venice Beach in ten minutes.

In Sand Francisco, you don't _want_ a car, and staying near Union Square will be expensive and a little boring for twenty somethings.

Good questions! but really as ackislander has said, we need a lot more information.

For example, are you all girls/boys? In other words, do you plan on all being in the same room? Or is it two girls and a boy? Or a boy with his girlfriend and his tagalong brother/sister? Put otherwise: one room or two? We wouldn't be able to give you any specific information until we can nail down what your needs are.

As Ackislander had already pointed out, public transportation is great in San Francisco and terrible to use in LA unless you have tons of time.

Are you looking at beaches for swimming or for wandering around? Beaches in California are usually too cold for swimming. If you are looking at beach activity, probably the best city to be in is San Diego. There's a lot of "beach" activity at Venice (LA), but it's not of the racing into water kind of activity, like you see in ads for Florida or the Caribbean.

Yes, SF Union Square hotels are fine, but mostly only on the east side. The west side abuts the Tenderloin district and you most likely don't want to be there.

Budget? That's an important question. How long do you plan on staying in each city?

So, let's dialog, OK?

Your best transportation between the cities is Amtrak IMO.
You get Amtrak bus transport from Fisherman's Wharf to Emeryville to connect with a train southbound.
You might want to stop in San Luis Obisbo or maybe Santa Barbara instead of going all the way to LAX in 1 day.
The Pacific Surfliner trains from SLO or SBA into LAX get a great view of miles of southern California beach. The Surfliners also go on into San Diego.

thank you so much for all your responses! well we are one boy and two girls, we are cousins so is not going to be anything romantic lol! im from florida and we will go there by plane that was my concern about the car bcause i dont think driving will be a good idea! about budget i dont really know.. in hotels not more than $120 per night or less than that? i dont know how is it over there we are not looking for any luxury hotels so a Best Western or anything like it will be fine for us! i dont know if renting a car will be too expensive.. or if we just rent it while in LA will be good? i dont know either on how many days is enough to spend a good time over there.. a week in between the two cities? is it a lot of time? btw, is the idea of going to las vegas is to crazy?? since we dont a car?? thank you

Coming from Florida It might be easier be easier to fly to Las Vegas and then rent a car to go from Las Vegas to southern California. Rental cars are cheaper in Las Vegas than in California. Return the car to Las Vegas and then fly to San Francisco. When you have seen SF then take the train back to LA so that you can see the coast from the train.

Travel between LA & SF without a car you shoud just fly. Amtrak is a huge time suck without any real savings. It's 9 hours from LA to SF - you wanna eat up an entire day sitting on a train?

AMTRAK - $58
As an example an advance purchase ticket on Southwest for June 5th is $49. 1 hour 15 minute plus you get an entire day to spend in SF.

You can then fly to Vegas. Or you can fly LA to Vegas & then Veags to SF. Gas prices in CA have been climbing again and they talk about high gas prices this summer so driving could cost you quite a bit if you rent a car and drive.

Right now Mega Bus is selling tix only thru April but if you bought now for the 7.5 hour bus trip from LA to SF the cost is $1. So in a few months they will release June dates and, depending on when you are actually coming for a visit, if you reserve early enough you can get a good deal. Mega Bus also goes from LA to Las Vegas.

http://us.megabus.com/Default.aspx

I can't help with hotels, but given your ages, for shopping I'd check out Abbot Kinney in Venice and the Robertson, Melrose and Melrose Place area of West Hollywood. You may also want to check out Malibu's Country Mart and Lumberyard if you're planning a beach day in Malibu. I'd definitely skip Pasadena. You can see the same chain stores at the 3rd Street Promenade in Santa Monica.