As long as you're staying in hostels, you'll have no shortage of people to hang out with. If you're on the shy side, you may have to push yourself a little to get out of your shell, but there won't be any shortage of people to hang out with. CB's five questions are spot on coversation starters. Another opener is "Could you pass the... (name of some item on the breakfast table)".
Group activities usually develop naturally around the breakfast or dinner table at the hostel. Along the lines of "I'm going to the ...., anyone want to come along?" Or "Anyone up for a drink?"
I think another thing that first time packers worry about is looking inexperienced, making the assumption that everyone else in the hostel is an experienced traveler. And that experienced travelers will make fun of a newbie. Hopefully by reading these boards you've figured out that most experienced travelers are passionate about it, and are both enthusiastic and generous about sharing their love of travel, adventure, etc. with anyone who expresses an interest. Another really great thing about travel vets is that most of us remember our feelings and insecurities when we took our first trips, and try to help others over those early challenges.
And it's amazing. When you first arrive, you are the greenest traveler in the hostel, and the next day, you're not.
If you're not staying in a hostel, I would think the best kind of tour to take would be a walking tour of the the city or a particular site in which you're interested. On a walking tour you can mix better with more people than if you're on a bus or boat or whatnot, because you're not pinned down to a particular seat. If you take a tour of a place you're particularly interested in, you'll probably meet people with similar interests, which gives you common ground to start with.
Good luck, and enjoy your trip!