If you're on public property, they're fair game. If you're standing on a public sidewalk, you are legally allowed to shoot. If you are shooting "news," you also won't need a release to sell the photo (celebrity photos count as news). Sometimes a production will "buy the street" and will be able to prohibit you from standing on the sidewalk they "bought."
I'm not paparazzi, but I'm often around a lot of celebrities in my TV job, and I've talked to a few paparazzi on occasion. I think a "typical" photo of an A-list celebrity would sell for anywhere from $100-$700 per use, depending on the event where taken (i.e., depends on how many other photographers also took the "same" picture), and the type of use (e.g., TV, I think, has to pay more than print).
However, there's really only a handful of "real" A-list celebrities: Britney Spears, Brad and Angelina, etc. Many people who you would think are "super-famous" (and they are), are actually B- and C-listers, and only command $25-$100 at most.
It's unique or unusual photos which get the big money (e.g., the first photos of an A-list celebrity couple's baby). Being first can command a fee up into six-figures. These are rare and extremely difficult to get.
However, should you get a decent photo of an A-lister, you would most likely sell it to an agency. The usual deal is they take 50%. A street candid of an A-lister working on a movie may only be worth $50-$100 at most, I'm just guessing (unless you've captured them doing something really out of the ordinary, and you're the only one that got it). So, if the photo sells for say, $100, you get $50. Where it starts to become worthwhile is if multiple outlets buy the photo rights. If ten outlets buy it, your $50 cut now becomes $500.
One of the agencies is "Splash." I think they're in Santa Monica. You can also sell directly to the tabloids--just buy a copy (or look online) and call the number for the photo desk or whatever. Selling directly just takes more leg work. Good luck!