Well if you are starting the conversation then I would say "hey what's up" because it's more casual and won't spook them. Then if they invite you to chat by saying hi back (and look like they actually want to talk) you can ask how their day is, try to be witty, throw in some jokes. If they are laughing, you have them in your grasp. If you have trouble with this, no problem, a lot of people are awkward and a lot of people think it's cute. You can say "haha sorry I'm so awkward" and they'll be like "haha it's fine" and then you can ask what they're into. Maybe they like video games or reading or digging for bones in their back yard but don't ask them how they got there. Just ask what they like until you find something you have in common. And remember to keep eye contact but not for too long you don't want to creep them out, but if you look away too much they will think you're not interested anymore. Avoid possible triggers like death and break ups because they might turn hostile and we don't want that. Just be yourself and have fun and if they don't want to be your friend well then that's their loss and you should probably take them out and hide their body behind the dumpster at Taco Bell. Happy socializing :)
