Two words: baby steps
Years ago I was a very introverted person. I rarely talked or gave my opinions and wasn't sure about many things.
But somehow, in the years that followed, I turned around a bit. I became more talkative, wasn't nearly as afraid to voice my opinions and became more confident.
This probably happened because I got to know the people around me, worked at tech support (meaning having to talk to dozens of strangers every day, even though it's mostly following steps and on the phone), and went to the pub more often.
This didn't happen overnight though. You can go and jump into the deep right away, and that might work too, but it also might not.
It seems that such personal development grows exponentially. Meaning that once you've taken one small step, it will be double as easy to take the next step. This makes sense because each little victory in your progress gives you a rush of confidence and pride.
So go ahead and say hi to people, just two letters, h i. And next time at the roundabout, take the exit. You're in a car, at reasonably low speeds, with an instructor, so it's not that dangerous, well, not for you anyway
Another approach is to put yourself in someone else's shoes. Pretend you're someone else and say 'hello'. You know when you've had a couple of drinks you suddenly talk so much (even to strangers!) you can hardly shut up? Well, that person is still you, only because of the alcohol, the mind doesn't do those excessive calculations that induce fear and ucertainty.
Maybe it helps thinking that way. I don't mean getting yourself drunk, or even acting that way, but to visualize flicking a switch in your head to disable or lower your uncertainty for a moment.
That technique has often helped me. When I wrote an e-mail to some high placed chief in the company but wasn't sure if I should sent it that way, I just turned off the power to my mind for a second and clicked 'send'. And guess what? It turned out to be just the right thing. Sometimes you just need to override your mind and go for it.