I was pretty shy / introverted in my early 20's. Got a job as a whitewater rafting guide, where I had to interact with new people every day (in a foreign language). It quickly goes away.

Another friend was terrified of public speaking so started doing stand up comedy, and now helps run one of the regular shows in a pub here.

You have to put in the hours to an extent, it's not just a case of trying a couple of times and giving up. Alcohol helps to a degree, but obviously don't go crazy with it. Probably hanging out with extroverts and taking note of what they do helps as well.