One of my favorite public photo events is Carnaval in San Francisco. I've attended Carnaval since 2008 and I can honestly say that if you want (or need) to practice photographing people, this is the place to do it. The music and dancing will make you want to samba in the streets, and the colorful costumes and floats beg to be photographed. The most important thing is that the participants expect to be photographed, making it so much easier for you.
If you're free to attend and would like to make some photographs, may I make some suggestions?
Get there early. I try to get there by 8:00 am so I can find a parking place as close as possible. I am not well versed in public transportation, but if you get off at the 24th street Bart Station, you'll be relatively close. I tend to drive my own car since I live in the city and it frees me up to do other things as soon as I'm done.
Wear comfortable shoes. You'll be on your feet a lot. Also, learn to "turn in a tight radius". When working in a crowd, keep your elbows in, and turn your head before you turn your body.
Keep moving, and keep watching. You need to develop of sense for spotting potential photographs before they actually occur. I tend to watch for interesting individuals that stand out in the crowd, children with a look of wonder, and costumes that are particularly colorful.
Watch the direction of the light. I try to avoid backlight (subject shadow pointing directly toward you) as the shadowed areas will usually be too underexposed to show much detail. If you expose for the shadows, the highlights will be too "hot" (overexposed) to yield any detail. To this end, I tend to walk with the sun over my shoulders, and wait for my subject to turn toward the light. You could argue that there might be a great photograph to be found when the sun is in your eyes and your subject's face is in the shadows. Maybe so, but I'll bet that you wind up deleting the image because the shadows aren't printable. Now if it’s a photo that I absolutely must have, I may supplement the shot with a speedlight. More on that later.
Simplify your camera settings. I generally set my cameras for aperture priority, ISO 400, cloudy white balance and a constant aperture of 5.6. I prefer this setting because the wider aperture yields a reduced depth of field that can help isolate the real center of interest. Also, a setting of 5.6 / ISO 400 is within the exposure range of most cameras. Using the time honored "sunny sixteen rule” to divine the proper, front lit exposure for this aperture/ISO combination in bright sunlight, you wind up with shutter speed of 1/3200, well within the range of most digital SLR cameras. With the lens still at 5.6, the proper open shade shutter speed would be 1/400, fast enough to stop almost any subject movement.
