You are seeing your ex today for the first time since you broke up what should you do?

Just be yourself. Smile when you see him, give him a quick hug if you want too. Nature will take it's course so don't push it and you just never know, you could get back together again. If this isn't the case then simply smile, say hello and ask how he's doing and move on. Act as if he's simply an aquaintance. Well, if its just a casual meeting to chat, then just chat about what has been going on in your life since the breakup and dont bring up your relationship with them or new ones, be vague about the new ones, if any. If its to rekindle, then you have to get all the reasons you broke up in the first place on the table, resolved and a plan for them not to happen again.