1) Pick a place. Let's say Paris.
2) Go to hostelworld.com, sort by highest review, choose the first hostel in your price range, book a week stay.
3) Sit at a large table in the common room, nurse a beer or coffee, work on your laptop, wait for others to wander by.
4) Strike up conversation, ask about where they've been, what they've done, and what they recommend.
I traveled for 3 months in SE Asia in this way. Everybody is in the same frame of mind as you, ready to meet new people, share stories.
It really is that simple.