I think you might be surprised how easy it is, its a matter of necessity to meet people to make the most of your trip. I'm also very introverted but I went to Australia on my own and I ended up meeting a lot of people.
First thing to know is that people that are staying on hostels are really friendly. Lots of people will say hi, introduce themselves and start up a conversation when you first see them in your dorm room or at a bulletin board. And no one is going to object to you doing the same, its a part of the whole experience so there is no pressure at all.
One thing I noticed was that I would gets lots of comments on a San Francisco tshirt I wore some times...I'm not from there, its just a souvenir from visiting a friend but it was the easiest conversation starter. Tons of people would say "oh hey, are you from california?" Now you've met someone