I assume you are after 'people pictures'.
One exercise we had to do in college was to head out to a public place and take 24 shots of people, asking each subject if we could take their picture -- no hiding behind mailboxes or anything -- we had to have their attention. Every one of us was scared to death, and it took me a whole day to finish the roll, with many breaks for coffee to recover my self-esteem after being blown off, but most of us managed to turn in decent contact sheets.
If you can manage that, you learn to get 'permission' with just a nod or a gesture, and you learn to shoot fast -- no fiddling with focus and exposure -- and that also makes you a better candid photographer for when the time calls for that.
Of the many lessons I learned that terrifying day was that if you look like you are on an assignment, and "working", most people consent. Another is that it seems to be far easier to get permission from a couple than it is from individuals, and better yet, a group of three friends.
If that suggestion seems out of line, here is another angle: There was a famous photographer who used to take someone with him and be pretending to shoot them, all the while shooting people passing by.
Another great thing is to attend public events, rallies, etc. It's great because people aren't just standing around, they are up to something and have something to say, AND they are expecting coverage.
Don't spread this around but I sometimes help myself by wearing my parking pass on a lanyard around my neck -- sorta kinda looks like a press pass at a distance. If it's a 'lefty' tree-hugging event, sometimes I go looking like I'm some scraggy photographer from the Daily Worker. A bit of subterfuge, but not much because I normally look that way most days.
Have courage!
