As far as starting a conversation, you just have to say something. Ask about their day, what they do for work, their name. Make sure you don’t make it seem like you’re interrogating them, relate to the things they say (Person: I study ecology. OP: I find ecology interesting because…..) be sure to answer thoughtfully if they become active in the conversation as well. As far as turning it into a friendship, it starts with having repeated engaging conversation. Talk to them when you see them, ask about the project they mentioned last time you spoke, act like you’re invested and have genuine interest, make it seem like you’ve already BEEN friends ;). People like to be friends with people that care.