Stranger Conversation Framework
Get good at compliments
Figure out what to compliment them on. This is a test for the conversation: If they just say thank you or very little, they don't want a conversation. If they talk more about the object of complement, etc. then they are a good candidate for continuing the conversation.
Small Talk
Ask them an easy question. How is your day going? What do you do? If they ask you the same question, you should have an interesting answer. Rehearse this before going out. Don't expect to have an interesting answer without planning for it.
Make the other person feel special
Active Listening. Care about their answers. Let them talk about themselves.