Hi I'm actually a really shy teenager right now so maybe I can help.
I think good things to ask are about what type of music they listen to, classes that they are taking, anything you might have in common, and actually politics or controversial issues, I know that sounds weird but even shy people have strong opinions and that may very well get them talking.
I usually rather be approached by only one person, more than 2 people usually freaks me out. And if someone asks me for help I will always help I like to be helpful and I think most people do too.
I think people are shy because they are afraid they wont be accepted, or they are intimidated by people who are not shy. A lot of my friends are shy at school and in public but and I happen to know that they aren't always shy. When me and my friends get together you would not think we were the same people, we act just like everyone else, it just takes more time for us to open up with someone, but once we do we are not shy around them anymore, we just have to build up some trust. I'm obviously not sure if this goes for all shy people but its true for me. I hope this helps you, and I think its great that you are trying to be friends with people who are shy, I know what its like to be ignored because I'm not the first one to talk.