Ask whoever owns the area of where you want to busk if you need a permit or anything.
The City of San Diego out here says that busking is not allowed anywhere, and yet, Balboa Park allows it on their premises- it's a hassle to find out and do it.
As much as I support theambitiouscard.com, I got my table for a WHOLE lot cheaper. Order a restaurant tray stand online, then find a woodshop and ask them to make you a table top. I got all of this for probably a little over 100. The table top is BEAUTIFUL and I have no complaints about my set up, it works fine!
Nice plug George

Nice plug Don

Get Cellini stuff, it's GOLD- stick with Cellini (but we won't get into that now will we?)
I'm 17 and have been busking for 2 years- just GETTING OUT THERE with an outline of a routine if the first step. THEN I formed my show. I had a few tricks I know I wanted to do, I scripted it out a bit, but then workshopping is being out on the street! Your routine will change SEVERAL times throughout... lord knows my show is so much better than when I started, but it's also MUCH different.
Listen to the older people here though, they know their stuff. Don is the nicest person ever- USE HIM....FOR ADVICE! He is the sole reason I am who I am as a busker today, the man has a heart of gold and cares about every single new magician (well... almost). George is incredible, just watch his stuff. I know JimmyTalksaLot pretty well- and he's GOOD. George is EVEN BETTER! (his words George, not mine). Listen to these people and you should do fine.
There is a stigma about a kid performing though in public. Be safe and bring a friend if need be- I know when I started I had one. DO NOT use Gazzo-esque jokes, use age appropriate jokes that fit you... not someone else.
Be yourself, have fun, SCRIPT OUT YOUR SHOW, and you'll do fine.
-Jeff T