First, you need to say where on the Earth you actually live. You do realize that this is the internet, right? You do realize that every country has different laws, right?
As for the U.S., you can take photos of people who are out in public places where privacy is not assumed. What you do with those images is another matter. If you want to use those images for commercial purposes, then you will need to get a model release from all recognizable people in the shot. If, however, you wan't to make fine-art prints of the same image, then you don't need permission or any signed documents. If you live outside of the United States, then you need to do more research as laws change from country to country.