It doesn't matter if people know your first name. Unless it's really really unique (it's not) there's absolutely no way for anyone to know who you are from just a first name. Even just a last name isn't helpful.
I assume your fear is more logical: that colleges, future employers, w/e will find it, not that police will. Say they google your first name. Say they SOMEHOW find that reddit topic. Say they click on your username and find your post where you said you may sell weed. Do you think they're going to throw out your application because some guy who shared your first name might have sold drugs a couple of years ago?
Even if your fear was less logical, and you think that the police saw your post on Reddit about how you might sell weed. Why would they waste time, energy, and resources investigating a guy who might sell weed at a low level? If they saw some random guy type "Hey, <name>," how would they even know if that person had any idea what they were talking about? Even if they decided that that was indeed your name, what would help them narrow that search down to you specifically?
