ask questions that require a sentance answer, not just yes/no. then pick out a detail from their sentance and ask about it. dont just constantly ask questions but try and relate your experiences to theirs, ask opinions etc.

for example

you:"so what did you do over summer?"
them"oh i went backpacking in europe"
then you could ask something like "which countries in europe did you vist?" or "who did you go with?" or "what was your favourite place?"... theres plenty of options that appear just from such a short reply. then you can just follow on from there. so, if you'd ask which countries they visited, try to relate something you know about those countries. so if theyd been to paris then you could say "oh i hear that the champs ellyses is amazing all lit up at night, what did you think of it?" or ask what theyd recommend you to do if you ever went there etc.

thats only a brief example but the possibilities are pretty much endless. if you find the person is still answering those questions with one word yes/no answers then maybe they dont feel like talking at that moment (in which case dont push too hard at continuing the conversation) or they might just be shy, in which case you can carry on asking questions and hopefully the person will open up a bit and overcome their shyness :)