How to open up to people?

I was wondering what are people's opinions on how to start a friendship, open up to people, how to know who you can trust>? Most importantly in my case how to open up to people when you have a really emotional past and you can't open up without crying? how to break a shell that seems unbreakable in social situations? Is it even right to open up? How to make close friends? how to become more outgoing?